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'''Natural language processing''' ('''NLP''') is a subfield of [[linguistics]], [[computer science]], and [[artificial intelligence]] concerned with the interactions between computers and human language, in particular how to program computers to process and analyze large amounts of [[natural language]] data.  The goal is a computer capable of "understanding" the contents of documents, including the [[context (language use)|contextual]] nuances of the language within them. The technology can then accurately extract information and insights contained in the documents as well as categorize and organize the documents themselves.

Challenges in natural language processing frequently involve [[speech recognition]], [[natural-language understanding]], and [[natural language generation|natural-language generation]].

== History ==
{{See|History of natural language processing}}

Natural language processing has its roots in the 1950s. Already in 1950, [[Alan Turing]] published an article titled "[[Computing Machinery and Intelligence]]" which proposed what is now called the [[Turing test]] as a criterion of intelligence, though at the time that was not articulated as a problem separate from artificial intelligence. The proposed test includes a task that involves the automated interpretation and generation of natural language.

=== Symbolic NLP (1950s – early 1990s) ===
The premise of symbolic NLP is well-summarized by [[John Searle]]'s [[Chinese room]] experiment: Given a collection of rules (e.g., a Chinese phrasebook, with questions and matching answers), the computer emulates natural language understanding (or other NLP tasks) by applying those rules to the data it confronts.

* '''1950s''': The [[Georgetown-IBM experiment|Georgetown experiment]] in 1954 involved fully [[automatic translation]] of more than sixty Russian sentences into English. The authors claimed that within three or five years, machine translation would be a solved problem.<ref>{{cite web|author=Hutchins, J.|year=2005|url=http://www.hutchinsweb.me.uk/Nutshell-2005.pdf|title=The history of machine translation in a nutshell}}{{self-published source|date=December 2013}}</ref>  However, real progress was much slower, and after the [[ALPAC|ALPAC report]] in 1966, which found that ten-year-long research had failed to fulfill the expectations, funding for machine translation was dramatically reduced.  Little further research in machine translation was conducted until the late 1980s when the first [[statistical machine translation]] systems were developed.
* '''1960s''': Some notably successful natural language processing systems developed in the 1960s were [[SHRDLU]], a natural language system working in restricted "[[blocks world]]s" with restricted vocabularies, and [[ELIZA]], a simulation of a [[Rogerian psychotherapy|Rogerian psychotherapist]], written by [[Joseph Weizenbaum]] between 1964 and 1966.  Using almost no information about human thought or emotion, ELIZA sometimes provided a startlingly human-like interaction. When the "patient" exceeded the very small knowledge base, ELIZA might provide a generic response, for example, responding to "My head hurts" with "Why do you say your head hurts?".
* '''1970s''': During the 1970s, many programmers began to write "conceptual [[ontology (information science)|ontologies]]", which structured real-world information into computer-understandable data.  Examples are MARGIE (Schank, 1975), SAM (Cullingford, 1978), PAM (Wilensky, 1978), TaleSpin (Meehan, 1976), QUALM (Lehnert, 1977), Politics (Carbonell, 1979), and Plot Units (Lehnert 1981).  During this time, the first [[chatterbots]] were written (e.g., [[PARRY]]).
* '''1980s''': The 1980s and early 1990s mark the heyday of symbolic methods in NLP. Focus areas of the time included research on rule-based parsing (e.g., the development of [[Head-driven phrase structure grammar|HPSG]] as a computational operationalization of [[generative grammar]]), morphology (e.g., two-level morphology<ref>{{citation|last=Koskenniemi|first=Kimmo|title=Two-level morphology: A general computational model of word-form recognition and production|url=http://www.ling.helsinki.fi/~koskenni/doc/Two-LevelMorphology.pdf|year=1983|publisher=Department of General Linguistics, [[University of Helsinki]]|author-link=Kimmo Koskenniemi}}</ref>), semantics (e.g., [[Lesk algorithm]]), reference (e.g., within Centering Theory<ref>Joshi, A. K., & Weinstein, S. (1981, August). [https://www.ijcai.org/Proceedings/81-1/Papers/071.pdf Control of Inference: Role of Some Aspects of Discourse Structure-Centering]. In ''IJCAI'' (pp. 385-387).</ref>) and other areas of natural language understanding (e.g., in the [[Rhetorical structure theory|Rhetorical Structure Theory]]). Other lines of research were continued, e.g., the development of chatterbots with [[Racter]] and [[Jabberwacky]]. An important development (that eventually led to the statistical turn in the 1990s) was the rising importance of quantitative evaluation in this period.<ref>{{Cite journal|last1=Guida|first1=G.|last2=Mauri|first2=G.|date=July 1986|title=Evaluation of natural language processing systems: Issues and approaches|journal=Proceedings of the IEEE|volume=74|issue=7|pages=1026–1035|doi=10.1109/PROC.1986.13580|s2cid=30688575|issn=1558-2256}}</ref>

=== Statistical NLP (1990s–2010s) ===
Up to the 1980s, most natural language processing systems were based on complex sets of hand-written rules.  Starting in the late 1980s, however, there was a revolution in natural language processing with the introduction of [[machine learning]] algorithms for language processing.  This was due to both the steady increase in computational power (see [[Moore's law]]) and the gradual lessening of the dominance of [[Noam Chomsky|Chomskyan]] theories of linguistics (e.g. [[transformational grammar]]), whose theoretical underpinnings discouraged the sort of [[corpus linguistics]] that underlies the machine-learning approach to language processing.<ref>Chomskyan linguistics encourages the investigation of "[[corner case]]s" that stress the limits of its theoretical models (comparable to [[pathological (mathematics)|pathological]] phenomena in mathematics), typically created using [[thought experiment]]s, rather than the systematic investigation of typical phenomena that occur in real-world data, as is the case in [[corpus linguistics]].  The creation and use of such [[text corpus|corpora]] of real-world data is a fundamental part of machine-learning algorithms for natural language processing.  In addition, theoretical underpinnings of Chomskyan linguistics such as the so-called "[[poverty of the stimulus]]" argument entail that general learning algorithms, as are typically used in machine learning, cannot be successful in language processing.  As a result, the Chomskyan paradigm discouraged the application of such models to language processing.</ref> 
*'''1990s''': Many of the notable early successes on statistical methods in NLP occurred in the field of [[machine translation]], due especially to work at IBM Research.  These systems were able to take advantage of existing multilingual [[text corpus|textual corpora]] that had been produced by the [[Parliament of Canada]] and the [[European Union]] as a result of laws calling for the translation of all governmental proceedings into all official languages of the corresponding systems of government.  However, most other systems depended on corpora specifically developed for the tasks implemented by these systems, which was (and often continues to be) a major limitation in the success of these systems. As a result, a great deal of research has gone into methods of more effectively learning from limited amounts of data.
*'''2000s''': With the growth of the web, increasing amounts of raw (unannotated) language data has become available since the mid-1990s. Research has thus increasingly focused on [[unsupervised learning|unsupervised]] and [[semi-supervised learning]] algorithms.  Such algorithms can learn from data that has not been hand-annotated with the desired answers or using a combination of annotated and non-annotated data.  Generally, this task is much more difficult than [[supervised learning]], and typically produces less accurate results for a given amount of input data.  However, there is an enormous amount of non-annotated data available (including, among other things, the entire content of the [[World Wide Web]]), which can often make up for the inferior results if the algorithm used has a low enough [[time complexity]] to be practical.

=== Neural NLP (present) ===
In the 2010s, [[representation learning]] and [[deep learning|deep neural network]]-style machine learning methods became widespread in natural language processing. That popularity was due partly to a flurry of results showing that such techniques<ref name="goldberg:nnlp17">{{cite journal |last=Goldberg |first=Yoav |year=2016 |arxiv=1807.10854 |title=A Primer on Neural Network Models for Natural Language Processing |journal=Journal of Artificial Intelligence Research |volume=57 |pages=345–420 |doi=10.1613/jair.4992 |s2cid=8273530 }}</ref><ref name="goodfellow:book16">{{cite book |first1=Ian |last1=Goodfellow |first2=Yoshua |last2=Bengio |first3=Aaron |last3=Courville |url=http://www.deeplearningbook.org/ |title=Deep Learning |publisher=MIT Press |year=2016 }}</ref> can achieve state-of-the-art results in many natural language tasks, e.g., in [[language modeling]]<ref name="jozefowicz:lm16">{{cite book |first1=Rafal |last1=Jozefowicz |first2=Oriol |last2=Vinyals |first3=Mike |last3=Schuster |first4=Noam |last4=Shazeer |first5=Yonghui |last5=Wu |year=2016 |arxiv=1602.02410 |title=Exploring the Limits of Language Modeling |bibcode=2016arXiv160202410J }}</ref> and parsing.<ref name="choe:emnlp16">{{cite journal |first1=Do Kook |last1=Choe |first2=Eugene |last2=Charniak |journal=Emnlp 2016 |url=https://aclanthology.coli.uni-saarland.de/papers/D16-1257/d16-1257 |title=Parsing as Language Modeling |access-date=2018-10-22 |archive-date=2018-10-23 |archive-url=https://web.archive.org/web/20181023034804/https://aclanthology.coli.uni-saarland.de/papers/D16-1257/d16-1257 |url-status=dead }}</ref><ref name="vinyals:nips15">{{cite journal |last1=Vinyals |first1=Oriol |last2=Kaiser |first2=Lukasz |display-authors=1 |journal=Nips2015 |title=Grammar as a Foreign Language |year=2014 |arxiv=1412.7449 |bibcode=2014arXiv1412.7449V |url=https://papers.nips.cc/paper/5635-grammar-as-a-foreign-language.pdf }}</ref> This is increasingly important [[artificial intelligence in healthcare|in medicine and healthcare]], where NLP helps analyze notes and text in [[Electronic health record|electronic health records]] that would otherwise be inaccessible for study when seeking to improve care.<ref>{{Cite journal|last1=Turchin|first1=Alexander|last2=Florez Builes|first2=Luisa F.|date=2021-03-19|title=Using Natural Language Processing to Measure and Improve Quality of Diabetes Care: A Systematic Review|journal=Journal of Diabetes Science and Technology|volume=15|issue=3|language=en|pages=553–560|doi=10.1177/19322968211000831|pmid=33736486|pmc=8120048|issn=1932-2968}}</ref>

== Methods: Rules, statistics, neural networks{{anchor|Statistical natural language processing (SNLP)}} ==
In the early days, many language-processing systems were designed by symbolic methods, i.e., the hand-coding of a set of rules, coupled with a dictionary lookup:<ref name=winograd:shrdlu71>{{cite thesis |last=Winograd |first=Terry |year=1971 |title=Procedures as a Representation for Data in a Computer Program for Understanding Natural Language |url=http://hci.stanford.edu/winograd/shrdlu/ }}</ref><ref name=schank77>{{cite book |first1=Roger C. |last1=Schank |first2=Robert P. |last2=Abelson |year=1977 |title=Scripts, Plans, Goals, and Understanding: An Inquiry Into Human Knowledge Structures |location=Hillsdale |publisher=Erlbaum |isbn=0-470-99033-3 }}</ref> such as by writing grammars or devising heuristic rules for [[stemming]].

More recent systems based on [[Machine learning|machine-learning]] algorithms have many advantages over hand-produced rules: 
* The learning procedures used during machine learning automatically focus on the most common cases, whereas when writing rules by hand it is often not at all obvious where the effort should be directed.
* Automatic learning procedures can make use of statistical inference algorithms to produce models that are robust to unfamiliar input (e.g. containing words or structures that have not been seen before) and to erroneous input (e.g. with misspelled words or words accidentally omitted). Generally, handling such input gracefully with handwritten rules, or, more generally, creating systems of handwritten rules that make soft decisions, is extremely difficult, error-prone and time-consuming.
* Systems based on automatically learning the rules can be made more accurate simply by supplying more input data. However, systems based on handwritten rules can only be made more accurate by increasing the complexity of the rules, which is a much more difficult task. In particular, there is a limit to the complexity of systems based on handwritten rules, beyond which the systems become more and more unmanageable. However, creating more data to input to machine-learning systems simply requires a corresponding increase in the number of man-hours worked, generally without significant increases in the complexity of the annotation process.

Despite the popularity of machine learning in NLP research, symbolic methods are still (2020) commonly used:
* when the amount of training data is insufficient to successfully apply machine learning methods, e.g., for the machine translation of low-resource languages such as provided by the [[Apertium]] system,
* for preprocessing in NLP pipelines, e.g., [[Tokenization (lexical analysis)|tokenization]], or
* for postprocessing and transforming the output of NLP pipelines, e.g., for [[knowledge extraction]] from syntactic parses.

=== Statistical methods ===
Since the so-called "statistical revolution"<ref name="johnson:eacl:ilcl09">[http://www.aclweb.org/anthology/W09-0103 Mark Johnson. How the statistical revolution changes (computational) linguistics.] Proceedings of the EACL 2009 Workshop on the Interaction between Linguistics and Computational Linguistics.</ref><ref name="resnik:langlog11">[http://languagelog.ldc.upenn.edu/nll/?p=2946 Philip Resnik. Four revolutions.] Language Log, February 5, 2011.</ref> in the late 1980s and mid-1990s, much natural language processing research has relied heavily on machine learning. The machine-learning paradigm calls instead for using [[statistical inference]] to automatically learn such rules through the analysis of large ''[[text corpus|corpora]]'' (the plural form of ''corpus'', is a set of documents, possibly with human or computer annotations) of typical real-world examples.

Many different classes of machine-learning algorithms have been applied to natural-language-processing tasks. These algorithms take as input a large set of "features" that are generated from the input data. Increasingly, however, research has focused on [[statistical models]], which make soft, [[probabilistic]] decisions based on attaching [[real-valued]] weights to each input feature (complex-valued [[word embedding|embeddings]],<ref>{{Cite web|title=Investigating complex-valued representation in NLP|url=https://wabyking.github.io/talks/mila-talk.pdf}}</ref> and neural networks in general have also been proposed, for e.g. speech<ref>{{cite arXiv|last1=Trabelsi|first1=Chiheb|last2=Bilaniuk|first2=Olexa|last3=Zhang|first3=Ying|last4=Serdyuk|first4=Dmitriy|last5=Subramanian|first5=Sandeep|last6=Santos|first6=João Felipe|last7=Mehri|first7=Soroush|last8=Rostamzadeh|first8=Negar|last9=Bengio|first9=Yoshua|last10=Pal|first10=Christopher J.|date=2018-02-25|title=Deep Complex Networks|class=cs.NE|eprint=1705.09792}}</ref>). Such models have the advantage that they can express the relative certainty of many different possible answers rather than only one, producing more reliable results when such a model is included as a component of a larger system.

Some of the earliest-used machine learning algorithms, such as [[decision tree]]s, produced systems of hard if-then rules similar to existing hand-written rules.  However, [[Part of speech tagging|part-of-speech tagging]] introduced the use of [[hidden Markov models]] to natural language processing, and increasingly, research has focused on statistical models, which make soft, probabilistic decisions based on attaching [[real-valued]] weights to the features making up the input data. The [[cache language model]]s upon which many [[speech recognition]] systems now rely are examples of such statistical models.  Such models are generally more robust when given unfamiliar input, especially input that contains errors (as is very common for real-world data), and produce more reliable results when integrated into a larger system comprising multiple subtasks.

Since the neural turn, statistical methods in NLP research have been largely replaced by neural networks. However, they continue to be relevant for contexts in which statistical interpretability and transparency is required.

=== Neural networks ===
{{Further|Artificial neural network}}
A major drawback of statistical methods is that they require elaborate feature engineering. Since 2015,<ref>{{Cite web|last=Socher|first=Richard|title=Deep Learning For NLP-ACL 2012 Tutorial|url=https://www.socher.org/index.php/Main/DeepLearningForNLP-ACL2012Tutorial|access-date=2020-08-17|website=www.socher.org}} This was an early Deep Learning tutorial at the ACL 2012 and met with both interest and (at the time) skepticism by most participants. Until then, neural learning was basically rejected because of its lack of statistical interpretability. Until 2015, deep learning had evolved into the major framework of NLP.</ref> the field has thus largely abandoned statistical methods and shifted to [[neural network]]s for machine learning. Popular techniques include the use of [[word embedding]]s to capture semantic properties of words, and an increase in end-to-end learning of a higher-level task (e.g., question answering) instead of relying on a pipeline of separate intermediate tasks (e.g., part-of-speech tagging and dependency parsing). In some areas, this shift has entailed substantial changes in how NLP systems are designed, such that deep neural network-based approaches may be viewed as a new paradigm distinct from statistical natural language processing. For instance, the term ''[[neural machine translation]]'' (NMT) emphasizes the fact that deep learning-based approaches to machine translation directly learn [[Seq2seq|sequence-to-sequence]] transformations, obviating the need for intermediate steps such as word alignment and language modeling that was used in [[statistical machine translation]] (SMT).

== Common NLP tasks ==
The following is a list of some of the most commonly researched tasks in natural language processing. Some of these tasks have direct real-world applications, while others more commonly serve as subtasks that are used to aid in solving larger tasks.

Though natural language processing tasks are closely intertwined, they can be subdivided into categories for convenience. A coarse division is given below.

=== Text and speech processing ===
; [[Optical character recognition]] (OCR)
:Given an image representing printed text, determine the corresponding text.

; [[Speech recognition]]: Given a sound clip of a person or people speaking, determine the textual representation of the speech.  This is the opposite of [[text to speech]] and is one of the extremely difficult problems colloquially termed "[[AI-complete]]" (see above).  In [[natural speech]] there are hardly any pauses between successive words, and thus [[speech segmentation]] is a necessary subtask of speech recognition (see below). In most spoken languages, the sounds representing successive letters blend into each other in a process termed [[coarticulation]], so the conversion of the [[analog signal]] to discrete characters can be a very difficult process. Also, given that words in the same language are spoken by people with different accents, the speech recognition software must be able to recognize the wide variety of input as being identical to each other in terms of its textual equivalent.
; [[Speech segmentation]]: Given a sound clip of a person or people speaking, separate it into words.  A subtask of [[speech recognition]] and typically grouped with it.

; [[Text-to-speech]]
:Given a text, transform those units and produce a spoken representation. Text-to-speech can be used to aid the visually impaired.<ref>{{Citation|last1=Yi|first1=Chucai|title=Assistive Text Reading from Complex Background for Blind Persons|date=2012|work=Camera-Based Document Analysis and Recognition|pages=15–28|publisher=Springer Berlin Heidelberg|language=en|citeseerx=10.1.1.668.869|doi=10.1007/978-3-642-29364-1_2|isbn=9783642293634|last2=Tian|first2=Yingli}}</ref>

; [[Word segmentation]] ([[Tokenization (lexical analysis)|Tokenization]])
:Separate a chunk of continuous text into separate words. For a language like [[English language|English]], this is fairly trivial, since words are usually separated by spaces. However, some written languages like [[Chinese language|Chinese]], [[Japanese language|Japanese]] and [[Thai language|Thai]] do not mark word boundaries in such a fashion, and in those languages text segmentation is a significant task requiring knowledge of the [[vocabulary]] and [[Morphology (linguistics)|morphology]] of words in the language. Sometimes this process is also used in cases like [[bag of words]] (BOW) creation in data mining.

=== Morphological analysis ===
; [[Lemmatisation|Lemmatization]]: The task of removing inflectional endings only and to return the base dictionary form of a word which is also known as a lemma. Lemmatization is another technique for reducing words to their normalized form. But in this case, the transformation actually uses a dictionary to map words to their actual form.<ref>{{Cite web|date=2020-12-06|title=What is Natural Language Processing? Intro to NLP in Machine Learning|url=https://www.gyansetu.in/what-is-natural-language-processing/|access-date=2021-01-09|website=GyanSetu!|language=en-US}}</ref>
; [[Morphology (linguistics)|Morphological segmentation]]: Separate words into individual [[morpheme]]s and identify the class of the morphemes. The difficulty of this task depends greatly on the complexity of the [[Morphology (linguistics)|morphology]] (''i.e.'', the structure of words) of the language being considered. [[English language|English]] has fairly simple morphology, especially [[inflectional morphology]], and thus it is often possible to ignore this task entirely and simply model all possible forms of a word (e.g., "open, opens, opened, opening") as separate words. In languages such as [[Turkish language|Turkish]] or [[Meitei language|Meitei]],<ref>{{cite journal |last1=Kishorjit |first1=N. |last2=Vidya |first2=Raj RK. |last3=Nirmal |first3=Y. |last4=Sivaji |first4=B. |year=2012 |url=http://aclweb.org/anthology//W/W12/W12-5008.pdf |title=Manipuri Morpheme Identification |journal=Proceedings of the 3rd Workshop on South and Southeast Asian Natural Language Processing (SANLP) |pages=95–108 |location=COLING 2012, Mumbai, December 2012 }}</ref> a highly [[Agglutination|agglutinated]] Indian language, however, such an approach is not possible, as each dictionary entry has thousands of possible word forms.
; [[Part-of-speech tagging]]: Given a sentence, determine the [[part of speech]] (POS) for each word. Many words, especially common ones, can serve as multiple parts of speech. For example, "book" can be a [[noun]] ("the book on the table") or [[verb]] ("to book a flight"); "set" can be a noun, verb or [[adjective]]; and "out" can be any of at least five different parts of speech.

; [[Stemming]]
:The process of reducing inflected (or sometimes derived) words to a base form (e.g., "close" will be the root for "closed", "closing", "close", "closer" etc.). Stemming yields similar results as lemmatization, but does so on grounds of rules, not a dictionary.

=== Syntactic analysis ===
; [[Grammar induction]]<ref>{{cite journal|last1=Klein|first1=Dan|last2=Manning|first2=Christopher D.|year=2002|title=Natural language grammar induction using a constituent-context model|url=http://papers.nips.cc/paper/1945-natural-language-grammar-induction-using-a-constituent-context-model.pdf|journal=Advances in Neural Information Processing Systems}}</ref>
: Generate a [[formal grammar]] that describes a language's syntax.
; [[Sentence breaking]] (also known as "[[sentence boundary disambiguation]]")
: Given a chunk of text, find the sentence boundaries. Sentence boundaries are often marked by [[Full stop|periods]] or other [[punctuation mark]]s, but these same characters can serve other purposes (e.g., marking [[abbreviation]]s).
; [[Parsing]]: Determine the [[parse tree]] (grammatical analysis) of a given sentence. The [[grammar]] for [[natural language]]s is [[ambiguous]] and typical sentences have multiple possible analyses: perhaps surprisingly, for a typical sentence there may be thousands of potential parses (most of which will seem completely nonsensical to a human). There are two primary types of parsing: ''dependency parsing'' and ''constituency parsing''. Dependency parsing focuses on the relationships between words in a sentence (marking things like primary objects and predicates), whereas constituency parsing focuses on building out the parse tree using a [[probabilistic context-free grammar]] (PCFG) (see also ''[[stochastic grammar]]'').

=== Lexical semantics (of individual words in context) ===
; [[Lexical semantics]]: What is the computational meaning of individual words in context?
; [[Distributional semantics]]: How can we learn semantic representations from data?
; [[Named entity recognition]] (NER): Given a stream of text, determine which items in the text map to proper names, such as people or places, and what the type of each such name is (e.g. person, location, organization). Although [[capitalization]] can aid in recognizing named entities in languages such as English, this information cannot aid in determining the type of [[named entity]], and in any case, is often inaccurate or insufficient.  For example, the first letter of a sentence is also capitalized, and named entities often span several words, only some of which are capitalized.  Furthermore, many other languages in non-Western scripts (e.g. [[Chinese language|Chinese]] or [[Arabic language|Arabic]]) do not have any capitalization at all, and even languages with capitalization may not consistently use it to distinguish names. For example, [[German language|German]] capitalizes all [[noun]]s, regardless of whether they are names, and [[French language|French]] and [[Spanish language|Spanish]] do not capitalize names that serve as [[adjective]]s.

; [[Sentiment analysis]] (see also [[Multimodal sentiment analysis]])
: Extract subjective information usually from a set of documents, often using online reviews to determine "polarity" about specific objects. It is especially useful for identifying trends of public opinion in social media, for marketing.
; [[Terminology extraction]]
:The goal of terminology extraction is to automatically extract relevant terms from a given corpus.
; [[Word-sense disambiguation]] (WSD): Many words have more than one [[Meaning (linguistics)|meaning]]; we have to select the meaning which makes the most sense in context.  For this problem, we are typically given a list of words and associated word senses, e.g. from a dictionary or an online resource such as [[WordNet]].
; [[Entity linking]]: Many words—typically proper names—refer to [[Named entity|named entities]]; here we have to select the entity (a famous individual, a location, a company, etc.) which is referred to in context.

=== Relational semantics (semantics of individual sentences) ===
; [[Relationship extraction]]: Given a chunk of text, identify the relationships among named entities (e.g. who is married to whom).
; [[Semantic parsing]]: Given a piece of text (typically a sentence), produce a formal representation of its semantics, either as a graph (e.g., in [[Abstract Meaning Representation|AMR parsing]]) or in accordance with a logical formalism (e.g., in [[Discourse representation theory|DRT parsing]]). This challenge typically includes aspects of several more elementary NLP tasks from semantics (e.g., semantic role labelling, word-sense disambiguation) and can be extended to include full-fledged discourse analysis (e.g., discourse analysis, coreference; see [[#Natural language understanding|Natural language understanding]] below).
; [[Semantic role labeling|Semantic role labelling]] (see also implicit semantic role labelling below)
:Given a single sentence, identify and disambiguate semantic predicates (e.g., verbal [[Frame semantics (linguistics)|frames]]), then identify and classify the frame elements ([[semantic roles]]).

=== Discourse (semantics beyond individual sentences) ===
; [[Coreference|Coreference resolution]]: Given a sentence or larger chunk of text, determine which words ("mentions") refer to the same objects ("entities"). [[Anaphora resolution]] is a specific example of this task, and is specifically concerned with matching up [[pronoun]]s with the nouns or names to which they refer. The more general task of coreference resolution also includes identifying so-called "bridging relationships" involving [[referring expression]]s. For example, in a sentence such as "He entered John's house through the front door", "the front door" is a referring expression and the bridging relationship to be identified is the fact that the door being referred to is the front door of John's house (rather than of some other structure that might also be referred to).
; [[Discourse analysis]]: This rubric includes several related tasks.  One task is discourse parsing, i.e., identifying the [[discourse]] structure of a connected text, i.e. the nature of the discourse relationships between sentences (e.g. elaboration, explanation, contrast).  Another possible task is recognizing and classifying the [[speech act]]s in a chunk of text (e.g. yes-no question, content question, statement, assertion, etc.).

; {{visible anchor|Implicit semantic role labelling}}
:Given a single sentence, identify and disambiguate semantic predicates (e.g., verbal [[Frame semantics (linguistics)|frames]]) and their explicit semantic roles in the current sentence (see [[#Semantic role labelling|Semantic role labelling]] above). Then, identify semantic roles that are not explicitly realized in the current sentence, classify them into arguments that are explicitly realized elsewhere in the text and those that are not specified, and resolve the former against the local text. A closely related task is zero anaphora resolution, i.e., the extension of coreference resolution to [[pro-drop language]]s.

; [[Textual entailment|Recognizing textual entailment]]: Given two text fragments, determine if one being true entails the other, entails the other's negation, or allows the other to be either true or false.<ref name="rte:11">PASCAL Recognizing Textual Entailment Challenge (RTE-7) https://tac.nist.gov//2011/RTE/</ref>

; [[Topic segmentation]] and recognition
:Given a chunk of text, separate it into segments each of which is devoted to a topic, and identify the topic of the segment.

; [[Argument mining]]
:The goal of argument mining is the automatic extraction and identification of argumentative structures from [[natural language]] text with the aid of computer programs.<ref>{{Cite journal|last1=Lippi|first1=Marco|last2=Torroni|first2=Paolo|date=2016-04-20|title=Argumentation Mining: State of the Art and Emerging Trends|url=https://dl.acm.org/doi/10.1145/2850417|journal=ACM Transactions on Internet Technology|language=en|volume=16|issue=2|pages=1–25|doi=10.1145/2850417|hdl=11585/523460|s2cid=9561587|issn=1533-5399}}</ref> Such argumentative structures include the premise, conclusions, the [[argument scheme]] and the relationship between the main and subsidiary argument, or the main and counter-argument within discourse.<ref>{{Cite web|title=Argument Mining - IJCAI2016 Tutorial|url=https://www.i3s.unice.fr/~villata/tutorialIJCAI2016.html|access-date=2021-03-09|website=www.i3s.unice.fr}}</ref><ref>{{Cite web|title=NLP Approaches to Computational Argumentation – ACL 2016, Berlin|url=http://acl2016tutorial.arg.tech/|access-date=2021-03-09|language=en-GB}}</ref>

=== Higher-level NLP applications ===
; [[Automatic summarization]] (text summarization): Produce a readable summary of a chunk of text.  Often used to provide summaries of the text of a known type, such as research papers, articles in the financial section of a newspaper.
; Book generation
:Not an NLP task proper but an extension of natural language generation and other NLP tasks is the creation of full-fledged books. The first machine-generated book was created by a rule-based system in 1984 (Racter, ''The policeman's beard is half-constructed'').<ref>{{Cite web|title=U B U W E B :: Racter|url=http://www.ubu.com/historical/racter/index.html|access-date=2020-08-17|website=www.ubu.com}}</ref> The first published work by a neural network was published in 2018, ''[[1 the Road]]'', marketed as a novel, contains sixty million words. Both these systems are basically elaborate but non-sensical (semantics-free) [[language model]]s. The first machine-generated science book was published in 2019 (Beta Writer, ''Lithium-Ion Batteries'', Springer, Cham).<ref>{{Cite book|last=Writer|first=Beta|date=2019|title=Lithium-Ion Batteries|language=en-gb|doi=10.1007/978-3-030-16800-1|isbn=978-3-030-16799-8|s2cid=155818532}}</ref> Unlike ''Racter'' and ''1 the Road'', this is grounded on factual knowledge and based on text summarization.
; [[Dialogue system|Dialogue management]]
:Computer systems intended to converse with a human.
; [[Document AI]]
:A Document AI platform sits on top of the NLP technology enabling users with no prior experience of artificial intelligence, machine learning or NLP to quickly train a computer to extract the specific data they need from different document types. NLP-powered Document AI enables non-technical teams to quickly access information hidden in documents, for example, lawyers, business analysts and accountants.<ref>{{Cite web|title=Document Understanding AI on Google Cloud (Cloud Next '19) - YouTube|url=https://www.youtube.com/watch?v=7dtl650D0y0| archive-url=https://ghostarchive.org/varchive/youtube/20211030/7dtl650D0y0| archive-date=2021-10-30|access-date=2021-01-11|website=www.youtube.com}}{{cbignore}}</ref>
;{{visible anchor|Grammatical error correction}}
:Grammatical error detection and correction involves a great band-width of problems on all levels of linguistic analysis (phonology/orthography, morphology, syntax, semantics, pragmatics). Grammatical error correction is impactful since it affects hundreds of millions of people that use or acquire English as a second language. It has thus been subject to a number of shared tasks since 2011.<ref>{{Cite web|last=Administration|title=Centre for Language Technology (CLT)|url=https://www.mq.edu.au/research/research-centres-groups-and-facilities/innovative-technologies/centres/centre-for-language-technology-clt|access-date=2021-01-11|website=Macquarie University|language=en-au}}</ref><ref>{{Cite web|title=Shared Task: Grammatical Error Correction|url=https://www.comp.nus.edu.sg/~nlp/conll13st.html|access-date=2021-01-11|website=www.comp.nus.edu.sg}}</ref><ref>{{Cite web|title=Shared Task: Grammatical Error Correction|url=https://www.comp.nus.edu.sg/~nlp/conll14st.html|access-date=2021-01-11|website=www.comp.nus.edu.sg}}</ref> As far as orthography, morphology, syntax and certain aspects of semantics are concerned, and due to the development of powerful neural language models such as [[GPT-2]], this can now (2019) be considered a largely solved problem and is being marketed in various commercial applications.
; [[Machine translation]]
:Automatically translate text from one human language to another.  This is one of the most difficult problems, and is a member of a class of problems colloquially termed "[[AI-complete]]", i.e. requiring all of the different types of knowledge that humans possess (grammar, semantics, facts about the real world, etc.) to solve properly.
; [[Natural language generation|Natural-language generation]]<nowiki> (NLG):</nowiki>
:Convert information from computer databases or semantic intents into readable human language.
; [[Natural-language understanding]] (NLU): Convert chunks of text into more formal representations such as [[first-order logic]] structures that are easier for [[computer]] programs to manipulate. Natural language understanding involves the identification of the intended semantic from the multiple possible semantics which can be derived from a natural language expression which usually takes the form of organized notations of natural language concepts. Introduction and creation of language metamodel and ontology are efficient however empirical solutions. An explicit formalization of natural language semantics without confusions with implicit assumptions such as [[closed-world assumption]] (CWA) vs. [[open-world assumption]], or subjective Yes/No vs. objective True/False is expected for the construction of a basis of semantics formalization.<ref>{{cite journal|last1=Duan|first1=Yucong|last2=Cruz|first2=Christophe|year=2011|title=Formalizing Semantic of Natural Language through Conceptualization from Existence|url=http://www.ijimt.org/abstract/100-E00187.htm|journal=International Journal of Innovation, Management and Technology|volume=2|issue=1|pages=37–42|archive-url=https://web.archive.org/web/20111009135952/http://www.ijimt.org/abstract/100-E00187.htm|archive-date=2011-10-09}}</ref>
; [[Question answering]]: Given a human-language question, determine its answer. Typical questions have a specific right answer (such as "What is the capital of Canada?"), but sometimes open-ended questions are also considered (such as "What is the meaning of life?").
; [[Text-to-image generation]]: Given a description of an image, generate an image that matches the description.<ref>{{Cite web |last=Robertson |first=Adi |date=2022-04-06 |title=OpenAI's DALL-E AI image generator can now edit pictures, too |url=https://www.theverge.com/2022/4/6/23012123/openai-clip-dalle-2-ai-text-to-image-generator-testing |access-date=2022-06-07 |website=The Verge |language=en}}</ref>
; Text-to-scene generation: Given a description of a scene, generate a [[3D model]] of the scene.<ref>{{Cite web |title=The Stanford Natural Language Processing Group |url=https://nlp.stanford.edu/projects/text2scene.shtml |access-date=2022-06-07 |website=nlp.stanford.edu}}</ref><ref>{{Cite journal |last1=Coyne |first1=Bob |last2=Sproat |first2=Richard |date=2001-08-01 |title=WordsEye: an automatic text-to-scene conversion system |url=https://doi.org/10.1145/383259.383316 |journal=Proceedings of the 28th Annual Conference on Computer Graphics and Interactive Techniques |series=SIGGRAPH '01 |location=New York, NY, USA |publisher=Association for Computing Machinery |pages=487–496 |doi=10.1145/383259.383316 |isbn=978-1-58113-374-5|s2cid=3842372 }}</ref>

== General tendencies and (possible) future directions ==
Based on long-standing trends in the field, it is possible to extrapolate future directions of NLP. As of 2020, three trends among the topics of the long-standing series of CoNLL Shared Tasks can be observed:<ref>{{Cite web|title=Previous shared tasks {{!}} CoNLL|url=https://www.conll.org/previous-tasks|access-date=2021-01-11|website=www.conll.org}}</ref>

* Interest on increasingly abstract, "cognitive" aspects of natural language (1999–2001: shallow parsing, 2002–03: named entity recognition, 2006–09/2017–18: dependency syntax, 2004–05/2008–09 semantic role labelling, 2011–12 coreference, 2015–16: discourse parsing, 2019: semantic parsing).
* Increasing interest in multilinguality, and, potentially, multimodality (English since 1999; Spanish, Dutch since 2002; German since 2003; Bulgarian, Danish, Japanese, Portuguese, Slovenian, Swedish, Turkish since 2006; Basque, Catalan, Chinese, Greek, Hungarian, Italian, Turkish since 2007; Czech since 2009; Arabic since 2012; 2017: 40+ languages; 2018: 60+/100+ languages)
* Elimination of symbolic representations (rule-based over supervised towards weakly supervised methods, representation learning and end-to-end systems)

=== Cognition and NLP ===
Most higher-level NLP applications involve aspects that emulate intelligent behaviour and apparent comprehension of natural language. More broadly speaking, the technical operationalization of increasingly advanced aspects of cognitive behaviour represents one of the developmental trajectories of NLP (see trends among CoNLL shared tasks above).

[[Cognition]] refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses."<ref>{{Cite web|title=Cognition|url=https://www.lexico.com/definition/cognition|archive-url=https://web.archive.org/web/20200715113427/https://www.lexico.com/definition/cognition|url-status=dead|archive-date=July 15, 2020|website=Lexico|publisher=[[Oxford University Press]] and [[Dictionary.com]]|access-date=6 May 2020}}</ref> [[Cognitive science]] is the interdisciplinary, scientific study of the mind and its processes.<ref>{{cite web|quote=Cognitive science is an interdisciplinary field of researchers from Linguistics, psychology, neuroscience, philosophy, computer science, and anthropology that seek to understand the mind. |url = http://www.aft.org/newspubs/periodicals/ae/summer2002/willingham.cfm |title= Ask the Cognitive Scientist|website = American Federation of Teachers|date = 8 August 2014 }}</ref> [[Cognitive linguistics]] is an interdisciplinary branch of linguistics, combining knowledge and research from both psychology and linguistics.<ref>{{Cite book|title=Handbook of Cognitive Linguistics and Second Language Acquisition|last=Robinson|first=Peter|publisher=Routledge|year=2008|isbn=978-0-805-85352-0|pages=3–8}}</ref> Especially during the age of [[#Symbolic NLP (1950s - early 1990s)|symbolic NLP]], the area of computational linguistics maintained strong ties with cognitive studies.

As an example, [[George Lakoff]] offers a methodology to build natural language processing (NLP) algorithms through the perspective of cognitive science, along with the findings of [[cognitive linguistics]],<ref>{{Cite book|title=Philosophy in the Flesh: The Embodied Mind and Its Challenge to Western Philosophy; Appendix: The Neural Theory of Language Paradigm |last= Lakoff |first= George |publisher= New York Basic Books|year=1999|isbn=978-0-465-05674-3|pages=569–583}}</ref> with two defining aspects:

# Apply the theory of [[conceptual metaphor]], explained by Lakoff as "the understanding of one idea, in terms of another" which provides an idea of the intent of the author.<ref>{{Cite book|title=A Cognitive Theory of Cultural Meaning|last= Strauss |first= Claudia |publisher= Cambridge University Press|year=1999|isbn=978-0-521-59541-4|pages=156–164}}</ref> For example, consider the English word ''big''. When used in a comparison ("That is a big tree"), the author's intent is to imply that the tree is ''physically large'' relative to other trees or the authors experience.  When used metaphorically ("Tomorrow is a big day"), the author's intent to imply ''importance''.  The intent behind other usages, like in "She is a big person", will remain somewhat ambiguous to a person and a cognitive NLP algorithm alike without additional information.
# Assign relative measures of meaning to a word, phrase, sentence or piece of text based on the information presented before and after the piece of text being analyzed, e.g., by means of a [[probabilistic context-free grammar]] (PCFG). The mathematical equation for such algorithms is presented in {{cite patent |country=US |number=9269353|status=patent}}:
::<math> {RMM(token_N)}
=
{PMM(token_N)} 
\times  
\frac{1}{2d}
\left (\sum_{i=-d}^d {((PMM(token_{N-1})} \times {PF(token_N,token_{N-1}))_i}\right )
</math>

::''Where,''
:::'''RMM''', is the Relative Measure of Meaning
:::'''token''', is any block of text, sentence, phrase or word
:::'''N''', is the number of tokens being analyzed
:::'''PMM''', is the Probable Measure of Meaning based on a corpora
:::'''d''', is the location of the token along the sequence of '''N-1''' tokens
:::'''PF''', is the Probability Function specific to a language

Ties with cognitive linguistics are part of the historical heritage of NLP, but they have been less frequently addressed since the statistical turn during the 1990s. Nevertheless, approaches to develop cognitive models towards technically operationalizable frameworks have been pursued in the context of various frameworks, e.g., of cognitive grammar,<ref>{{Cite web|title=Universal Conceptual Cognitive Annotation (UCCA)|url=https://universalconceptualcognitiveannotation.github.io/|access-date=2021-01-11|website=Universal Conceptual Cognitive Annotation (UCCA)|language=en-US}}</ref> functional grammar,<ref>Rodríguez, F. C., & Mairal-Usón, R. (2016). [https://www.redalyc.org/pdf/1345/134549291020.pdf Building an RRG computational grammar]. ''Onomazein'', (34), 86-117.</ref> construction grammar,<ref>{{Cite web|title=Fluid Construction Grammar – A fully operational processing system for construction grammars|url=https://www.fcg-net.org/|access-date=2021-01-11|language=en-US}}</ref> computational psycholinguistics and cognitive neuroscience (e.g., [[ACT-R]]), however, with limited uptake in mainstream NLP (as measured by presence on major conferences<ref>{{Cite web|title=ACL Member Portal {{!}} The Association for Computational Linguistics Member Portal|url=https://www.aclweb.org/portal/|access-date=2021-01-11|website=www.aclweb.org}}</ref> of the [[Association for Computational Linguistics|ACL]]). More recently, ideas of cognitive NLP have been revived as an approach to achieve [[Explainable artificial intelligence|explainability]], e.g., under the notion of "cognitive AI".<ref>{{Cite web|title=Chunks and Rules|url=https://www.w3.org/Data/demos/chunks/chunks.html|access-date=2021-01-11|website=www.w3.org|language=en}}</ref> Likewise, ideas of cognitive NLP are inherent to neural models [[multimodal interaction|multimodal]] NLP (although rarely made explicit).<ref>{{Cite journal|doi=10.1162/tacl_a_00177|title=Grounded Compositional Semantics for Finding and Describing Images with Sentences|year=2014|last1=Socher|first1=Richard|last2=Karpathy|first2=Andrej|last3=Le|first3=Quoc V.|last4=Manning|first4=Christopher D.|last5=Ng|first5=Andrew Y.|journal=Transactions of the Association for Computational Linguistics|volume=2|pages=207–218|s2cid=2317858|doi-access=free}}</ref>

== See also ==
{{Div col|colwidth=22em}}
* ''[[1 the Road]]''
* [[Automated essay scoring]]
* [[Biomedical text mining]]
* [[Compound term processing]]
* [[Computational linguistics]]
* [[Computer-assisted reviewing]]
* [[Controlled natural language]]
* [[Deep learning]]
* [[Deep linguistic processing]]
* [[Distributional semantics]]
* [[Foreign language reading aid]]
* [[Foreign language writing aid]]
* [[Information extraction]]
* [[Information retrieval]]
* [[Language and Communication Technologies]]
* [[Language technology]]
* [[Latent semantic indexing]]
* [[Native-language identification]]
* [[Natural-language programming]]
* [[Natural-language understanding]]
* [[Natural-language user interface|Natural-language search]]
* [[Outline of natural language processing]]
* [[Query expansion]]
* [[Query understanding]]
* [[Reification (linguistics)]]
* [[Speech processing]]
* [[Spoken dialogue systems]]
* [[Text-proofing]]
* [[Text simplification]]
* [[Transformer (machine learning model)]]
* [[Truecasing]]
* [[Question answering]]
* [[Word2vec]]
{{Div col end}}

== References ==
{{Reflist|30em}}

== Further reading ==
<!-- In alphabetical order of by last name -->
{{Refbegin}}
* {{Cite journal |last1=Bates |first1=M|year=1995|title=Models of natural language understanding|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=92|issue=22|pages=9977–9982|doi=10.1073/pnas.92.22.9977|pmid=7479812|pmc=40721|bibcode=1995PNAS...92.9977B|doi-access=free}}
* Steven Bird, Ewan Klein, and Edward Loper (2009). ''Natural Language Processing with Python''. O'Reilly Media. {{ISBN|978-0-596-51649-9}}.
* Daniel Jurafsky and James H. Martin (2008). ''Speech and Language Processing'', 2nd edition. Pearson Prentice Hall. {{ISBN|978-0-13-187321-6}}.
* Mohamed Zakaria Kurdi (2016). ''Natural Language Processing and Computational Linguistics: speech, morphology, and syntax'', Volume 1. ISTE-Wiley. {{ISBN|978-1848218482}}.
* Mohamed Zakaria Kurdi (2017). ''Natural Language Processing and Computational Linguistics: semantics, discourse, and applications'', Volume 2. ISTE-Wiley. {{ISBN| 978-1848219212}}.
* Christopher D. Manning, Prabhakar Raghavan, and Hinrich Schütze (2008). ''Introduction to Information Retrieval''. Cambridge University Press. {{ISBN|978-0-521-86571-5}}. [http://nlp.stanford.edu/IR-book/ Official html and pdf versions available without charge.]
* Christopher D. Manning and Hinrich Schütze (1999). ''Foundations of Statistical Natural Language Processing''. The MIT Press. {{ISBN|978-0-262-13360-9}}.
* David M. W. Powers and Christopher C. R. Turk (1989). ''Machine Learning of Natural Language''. Springer-Verlag. {{ISBN|978-0-387-19557-5}}.
{{Refend}}

==External links==
* {{Commonscatinline}}

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