A method by which terms or subject headings from a controlled vocabulary are selected by a human or computer to represent the concepts in or attributes of a content object. The terms may or may not occur in the content object. (ANSI/NISO Z39.19-2005)

An operation intended to represent the results of the content analysis of a document by means of a controlled indexing language or by natural language. [ISO 5127/1] (ANSI/NISO Z.39.19-2005)

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.