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In linguistics, a corpus (plural corpora) or text corpus is a large and structured set of texts (nowadays usually electronically stored and processed). They are used to do statistical analysis and hypothesis testing, checking occurrences or validating linguistic rules within a specific language territory.
A corpus may contain texts in a single language (monolingual corpus) or text data in multiple languages (multilingual corpus). Multilingual corpora that have been specially formatted for side-by-side comparison are called aligned parallel corpora.
In order to make the corpora more useful for doing linguistic research, they are often subjected to a process known as annotation. An example of annotating a corpus is part-of-speech tagging, or POS-tagging, in which information about each word's part of speech (verb, noun, adjective, etc.) is added to the corpus in the form of tags. Another example is indicating the lemma (base) form of each word. When the language of the corpus is not a working language of the researchers who use it, interlinear glossing is used to make the annotation bilingual.
Some corpora have further structured levels of analysis applied. In particular, a number of smaller corpora may be fully parsed. Such corpora are usually called Treebanks or Parsed Corpora. The difficulty of ensuring that the entire corpus is completely and consistently annotated means that these corpora are usually smaller, containing around one to three million words. Other levels of linguistic structured analysis are possible, including annotations for morphology, semantics and pragmatics.
Corpora are the main knowledge base in corpus linguistics. The analysis and processing of various types of corpora are also the subject of much work in computational linguistics, speech recognition and machine translation, where they are often used to create hidden Markov models for part of speech tagging and other purposes. Corpora and frequency lists derived from them are useful for language teaching. Corpora can be considered as a type of foreign language writing aid as the contextualised grammatical knowledge acquired by non-native language users through exposure to authentic texts in corpora allows learners to grasp the manner of sentence formation in the target language, enabling effective writing.
Text corpora are also used in the study of historical documents, for example in attempts to decipher ancient scripts, or in Biblical scholarship. Some archaeological corpora can be of such short duration that they provide a snapshot in time. One of the shortest corpora in time, may be the 15–30 year Amarna letters texts (1350 BC). The corpus of an ancient city, (for example the "Kültepe Texts" of Turkey), may go through a series of corpora, determined by their find site dates.
Some notable text corpora
- Google N-Grams Corpus – Largest English corpus at 155 billion words. Also has corpora for other languages.
- American National Corpus
- Bank of English
- British National Corpus
- Corpus Juris Secundum
- Corpus of Contemporary American English (COCA) 425 million words, 1990–2011. Freely searchable online.
- Brown Corpus, forming part of the "Brown Family" of corpora, together with LOB, Frown and F-LOB.
- International Corpus of English
- Oxford English Corpus
- Scottish Corpus of Texts & Speech
- European languages
- Bulgarian National Corpus
- Croatian Language Corpus
- Croatian National Corpus
- Czech National Corpus
- Russian National Corpus
- Slovenian National Corpus
- Thesaurus Linguae Graecae (Ancient Greek)
- Eastern Armenian National Corpus (EANC) 110 million words. Freely searchable online.
- National Corpus of Polish
- German Reference Corpus (DeReKo) More than 4 billion words of contemporary written German.
- Spanish text corpus by Molino de Ideas, which contains 660 millions words.(Spanish)
- CorALit: the Corpus of Academic Lithuanian Academic texts published in 1999–2009 (approx. 9 million words). Compiled at the University of Vilnius, Lithuania 
- Turkish National Corpus 
- Middle Eastern Languages
- Hamshahri Corpus (Persian a.k.a. Farsi)
- Amarna letters, (for Akkadian, Egyptian, Sumerogram's, etc.)
- TEP: Tehran English-Persian Parallel Corpus
- TMC: Tehran Monolingual Corpus, Standard corpus for Persian Language Modeling
- Bijankhan Corpus A Contemporary Persian Corpus for NLP researches
- Neo-Assyrian Text Corpus Project
- Quranic Arabic Corpus (Classical Arabic)
- Parallel Corpus of Diverse Languages
- East Asian Languages
- Kotonoha Japanese language corpus
- Corpus linguistics
- Linguistic Data Consortium
- Natural language processing
- Natural Language Toolkit
- Parallel text alignment
- Search engines: they access the "web corpus".
- Speech corpus
- Translation memory
- Zipf's Law
- Yoon, H., & Hirvela, A. (2004). ESL Student Attitudes toward Corpus Use in L2 Writing. Journal Of Second Language Writing, 13(4), 257–283. Retrieved 21 March 2012.
- tatoeba statistics
- Free, web-based corpora (45-425 million words each): American (COCA, COHA, TIME), British (BNC), Spanish, Portuguese
- Computational Linguistics on the Open Directory Project
- ACL SIGLEX Resource Links: Text Corpora
- The Leipzig Glossing Rules: Conventions for interlinear morpheme-by-morpheme glosses
- Developing Linguistic Corpora: a Guide to Good Practice
- An interface for querying automatically-constructed virtual corpora.
- TEP: Tehran English-Persian Parallel Corpus.
-  Building synchronous parallel corpora of the languages taught at the Faculty of Arts of Charles University.
- TS Corpus - A Turkish Corpus freely available for academic research.