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

Most thesauri do not include definitions, but many dictionaries include listings of synonyms. 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. The word 'thesaurus' was used in 1852 by Peter Mark Roget for his Roget's Thesaurus. Synonym dictionaries have a long history.

What is a synonym