Unlike dictionaries, a thesaurus does not contain definitions or pronunciations. Instead, if a person has a word in mind but it isn't quite right, they can look up the word in a thesaurus to find related or similar words. Conversely, it can be used to find something that is the opposite of a particular word.
The term is also used to describe a list of subject headings or descriptors assigned in a database, index, or catalog that can be used to search it. With either usage, there are both general and specialized versions.