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.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. Plural: thesauri. See also: controlled vocabulary, lead-in vocabulary, and metathesaurus.


Examples:
Art & Architecture Thesaurus (AAT)
GeoRef Thesaurus
Legislative Indexing Terms: The CRS Thesaurus
Thesaurus of ERIC Descriptors
Thesaurus of Psychological Index Terms
Thesaurus of Sociological Indexing Terms

For other examples of online subject thesauri, see the: Archaeological Objects Thesaurus, Astronomy Thesaurus, Biocomplexity Thesaurus, British Museum Materials Thesaurus, The Cook's Thesaurus, NAL Agricultural Thesaurus, Thesaurus for Graphic Materials, and Thesaurus of Musical Instruments. To learn more about thesauri, see Wikipedia.


From ODLIS, Online Dictionary of Library and Information Science by Joan M. Reitz
http://lu.com/odlis/odlis_c.cfm