Thesaurus

Synonym directory search engines use to help increase return relevancy. Thesaurus tools can also be used as a keyword research tool to help search marketers find related keywords to target.

Similar to a dictionary but contains a lists of synonyms rather than definitions.

A list of synonyms a search engine can use to find matches for particular words if the words themselves don't appear in documents.

A thesaurus stores synonyms and related words. This allows a search engine to map city planning to land use, for example, and show the relevant pages even if the vocabulary of the text did not match.