Subject headings are lists of terms - often quite lengthy to represent complex concepts - which are used for indexing for retrieval, and sometimes browsing. They will generally include synonyms, and sometimes hierarchical and 'SEE ALSO' relations. Complex subject heading lists can be very similar to thesauri. Whereas, the simpler forms are little more than lists of keywords. They are most commonly used in situations where only one heading, or only a few, are added to each record, e.g. library databases and bibliographies.
The most widely known and widely used terminology of this sort is the Library of Congress Subject Heading (LCSH) it provides subject indexing in many library databases, and increasingly in digital environments.