means A reference book that groups words by meaning, listing synonyms and related terms so you can find a better or more precise word than the one you started with.

from Straight from the Latin thesaurus, meaning "treasure" or "treasure-house" — itself borrowed from the Greek thesauros, a store or storehouse for precious things. So a thesaurus is literally a hoard of words. The modern sense was cemented by Peter Mark Roget, who in the 19th century published his famous Thesaurus of English Words and Phrases, turning the old word for a treasure-chest into the name for a vault of vocabulary.