THESAURUS [Plurals: traditionally thesauri, more recently and less formally thesauruses].
1. A work of REFERENCE presented as a treasure house (Greek thēsaurós) of information about words, such as Thomas Cooper's bilingual dictionary, the Thesaurus Linguae Romanae et Britannicae (‘Thesaurus of the Roman and British Languages’, 1565).

2. A work of reference containing lists of associated, usually undefined, words (such as synonyms) arranged thematically, in the style of ROGET's THESAURUS of English Words and Phrases (1852).

3. A work of reference containing such lists but presented alphabetically, such as The New Roget's Thesaurus in Dictionary Form ( Putnam's, 1961) and The Oxford Thesaurus (1991).

4. In information technology, an alphabetic index list of key terms, through which information of a specialist nature can be retrieved from a database.

5. In word processing, a stored list of synonyms and antonyms, to be consulted in the preparation of texts, and provided as a service comparable to a spelling checker.

the·sau·rus / [unvoicedth]əˈsôrəs/ • n. (pl. -sau·ri / -ˈsôrī/ or -sau·rus·es) a book that lists words in groups of synonyms and related concepts. ∎ archaic a dictionary or encyclopedia.

thesaurus A feature of word processing systems whereby similes and synonyms may be displayed on screen and incorporated into the text. Full text retrieval systems may have thesaurus searching as an option whereby terms similar in meaning to those sought will also be located. A thesaurus can be used to define a set of allowed terms for use as keywords during the entry of data into text retrieval systems.

thesaurus originally, a dictionary or encyclopedia; after the publication of Roget's Thesaurus, the meaning narrowed to its current sense of a book that lists words in groups of synonyms and related concepts. Recorded in English from the late 16th century, the word comes via Latin from Greek thēsauros ‘storehouse, treasure’.

thesaurus treasury, spec. of knowledge. XIX. — L. thēsaurus TREASURE — Gr. thēsaurós.