A controlled vocabulary or classification system and the rules for its application. An indexing language is used for the representation of concepts dealt with in documents [content objects] and for the retrieval of such documents [content objects] from an information storage and retrieval system. [ISO 5127/1] (ANSI/NISO Z39.19-2005)

National Information Standards Organization. (2005). Guidelines for the construction, format, and management of monolingual controlled vocabularies (ANSI/NISO Z39.19-2005). Bethesda, MD: NISO Press.