Subject heading
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Definition
A subject heading is part of a systematic list of terms that describe a given subject matter, e.g. like in a library catalogue.
See also: pedagogical vocabularies for an overview of other controlled vocabularies.
According to Reitz, a subject heading is “The most specific word or phrase that describes the subject, or one of the subjects, of a work, selected from a list of preferred terms (controlled vocabulary) and assigned as an added entry in the bibliographic record to serve as an access point in the library catalog. A subject heading may be subdivided by the addition of subheadings (example: Libraries--History--20th century) or include a parenthetical qualifier for semantic clarification, as in Mice (Computers).” (retrieved 13:56, 27 February 2009 (UTC))
The library of congress uses the term "authority record" to describe the “tool used by librarians to establish forms of names (for persons, places, meetings, and organizations), titles, and subjects used on bibliographic records. Authority records enable librarians to provide uniform access to materials in library catalogs and to provide clear identification of authors and subject headings. For example, works about "movies," "motion pictures," "cinema," and "films" are all entered under the established subject heading "Motion pictures."”.
An example
The library of congress authorities returns many entreis for an "educational technology" "Subject authority headings" search. However, only a subset authorized headings, e.g. formally established in a retrievable authority record. The main "educational technology" term (without subheadings) identifies the following synomymes (if I understood right): Instructional technology, Technology in education. It then defines "see also", such as "Audio-visual education" and parent terms such as Technology, Educational innovation, instructional systesm, teaching - aids and devices.
Standards
Subject headings can be quite formal, e.g. see Marc 21 Format for Authority Data's topical terms (X50)
Bibliography
Reitz, J.M. Online Dictionary for Library and Information Science, http://lu.com/odlis/odlis_g.cfm