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 [an] idea may be most fitly and aptly expressed" – to quote
Peter Mark Roget, architect of the best known thesaurus in the English language.