1. An organized arrangement of words and phrases used to index content and/or to retrieve content through browsing or searching. It typically includes preferred and variant terms and has a defined scope or describes a specific domain.(from Introduction to Controlled Vocabularies: Terminology for Art, Architecture, and Other Cultural Works, "What Are Controlled Vocabularies?", Patricia Harpring.
2. A collection of preferred terms that are used to promote consistent description and retrieval of data. (from Building Ontologies with Basic Formal Ontology, R. Arp, B. Smith, A. Spear)