A book — or now more often a website or a button in your word processor — that groups words by meaning. You look up a word you already know, and it shows you other words that mean something close: for 'happy' it might give glad, pleased, cheerful, delighted, content. Writers use one when a word feels too plain, or when they have used the same word three times in one paragraph. Note the difference from a dictionary: a dictionary tells you what a word means, a thesaurus tells you what else you could say instead. The plural is unusual — careful writers say 'thesauri', but 'thesauruses' is common and fine. The most famous one is Roget's Thesaurus, and older speakers may simply say 'look it up in Roget's'.