A controlled vocabulary, also called an authority file, is an authoritative list of terms to be used in indexing (human or automated). A controlled vocabulary for a project might actually include multiple authority files for different kinds of terms.

Controlled vocabularies are used to ensure consistent indexing, particularly when indexing multiple documents, periodical articles, web pages or sites, etc. They may also be used when indexing a single work, such as a encyclopedia, by multiple indexers.

Controlled vocabularies do not necessarily have any structure or relationships between terms within the list. Controlled vocabularies are often used for name authorities (proper nouns), such as persons, organization names, company names, etc.

Online controlled vocabularies often have synonyms or See references to point the user or search engine from an incorrect (unpreferred) variant to the equivalent preferred term in the controlled vocabulary. However, this is not a required attribute of a controlled vocabulary.

Controlled vocabularies are the broadest category, which includes thesauri and taxonomies. Thesauri and taxonomies are specific kinds of controlled vocabularies, but not all controlled vocabularies are thesauri or taxonomies.