In general usage, a thesaurus is a reference work that lists words grouped together according to similarity of meaning (containing synonyms and sometimes antonyms), in contrast to a dictionary, which provides definitions for words, and generally lists them in alphabetical order. The main purpose of such reference works is to help the user "to find the word, or words, by which () idea may be most fitly and aptly expressed" – to quote Peter Mark Roget, architect of the best known thesaurus in the English language.〔Roget, Peter. 1852. ''Thesaurus of English Language Words and Phrases''〕
Although including synonyms, a thesaurus should not be taken as a complete list of all the synonyms for a particular word. The entries are also designed for drawing distinctions between similar words and assisting in choosing exactly the right word. Unlike a dictionary, a thesaurus entry does not give the definition of words.
In library science and information science thesauri have been widely used to specify domain models. Recently, thesauri have been implements with SKOS.
The word "thesaurus" is derived from 16th-century New Latin, in turn from Latin ''thēsaurus'', which is the Latinisation of the Greek (''thēsauros''), "treasure, treasury, storehouse".〔("thesaurus" ). ''Online Etymology Dictionary''.〕 The word ''thēsauros'' is of uncertain etymology. Douglas Harper derives it from the root of the Greek verb τιθέναι ''tithenai'', "to put, to place."〔 Robert Beekes rejected an Indo-European derivation and suggested a Pre-Greek suffix .〔R. S. P. Beekes, ''Etymological Dictionary of Greek'', Brill, 2009, p. 548.〕
From the 16th to the 19th centuries, the term "thesaurus" might be applied to any dictionary or encylopedia; as in the ''Thesaurus linguae latinae(1532)'', and the ''Thesaurus linguae graecae(1572)''. The meaning "collection of words arranged according to sense" is first attested in 1852 in Roget's title and ''thesaurer'' is attested in Middle English for "treasurer".〔
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