A thesaurus (plural thesauri or thesauruses), sometimes called a synonym dictionary or dictionary of synonyms, is a reference work which arranges words by their meanings, sometimes as a hierarchy of broader and narrower terms, sometimes simply as lists of synonyms and antonyms. They are often used by writers to help find the best word to express an idea:

...to find the word, or words, by which [an] idea may be most fitly and aptly expressed

Synonym dictionaries have a long history. The word 'thesaurus' was used in 1852 by Peter Mark Roget for his Roget's Thesaurus.

While some works called "thesauri", such as Roget's Thesaurus, group words in a hierarchical hypernymic taxonomy of concepts, others are organized alphabetically or in some other way.

Most thesauri do not include definitions, but many dictionaries include listings of synonyms.

Some thesauri and dictionary synonym notes characterize the distinctions between similar words, with notes on their "connotations and varying shades of meaning". Some synonym dictionaries are primarily concerned with differentiating synonyms by meaning and usage. Usage manuals such as Fowler's Dictionary of Modern English Usage or Garner's Modern English Usage often prescribe appropriate usage of synonyms.

Writers sometimes use thesauri to avoid repetition of words — elegant variation — which is often criticized by usage manuals: "writers sometimes use them not just to vary their vocabularies but to dress them up too much".