Thesaurus Day is celebrated on January 18, 2018. In general usage, a thesaurus is a reference work that lists words grouped together according to similarity of meaning, 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 Roget, architect of the best known thesaurus in the English language.
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. (With material from: Wikipedia)