Thesaurus in the United States

A Thesaurus is an online database or a book of synonyms and near-synonyms in a written language, usually arranged conceptually, although dictionary arrangement is not uncommon. The first thesaurus of the English language, published in 1852, was compiled by Peter Mark Roget. For an online thesaurus of the English language, see Merriam-Webster OnLine. See also: crossword puzzle dictionary.
Also refers to an alphabetically arranged lexicon of terms comprising the specialized vocabulary of an academic discipline or field of study, showing the logical and semantic relations among terms, particularly a list of subject headings or descriptors used as preferred terms in indexing the literature of the field. In information retrieval, a thesaurus can be used to locate broader terms and related terms if the user wishes to expand retrieval, or narrower terms to make a search statement more specific. A well-designed thesaurus also enables the indexer to maintain consistency in the assignment of indexing terms to documents.

Controlled vocabulary (an established list of preferred terms from which a cataloger or indexer must select when assigning subject headings or descriptors in a bibliographic record, to indicate the content of the work in a library catalog, index, or bibliographic database) is usually listed alphabetically in a subject headings list or thesaurus of indexing terms.

The plural of Thesaurus is thesauri.

Example of a well known Thesaurus is the Legislative Indexing Terms: The CRS Thesaurus.

A metathesaurus is an alphabetically arranged list of the specialized vocabulary of an academic discipline or group of related disciplines, indicating the semantic relations between terms, designed to integrate a number of separate controlled vocabularies (thesauri) developed independently to facilitate information retrieval