Natural or everyday language frequently provides more than one word that is representative of a particular concept. For example, Afro Americans, Blacks, and Negroes are similar terms representing the concept, African Americans. In many databases, similar terms with similar meanings are grouped together under a single subject term (see the example below).
Database companies employ indexers that assign subjects terms to each document in a database. This insures that all documents about the same topic will be assigned the same subject term, regardless of the keywords and natural language used in individual articles. Searching by subject includes similar terms.
The library's catalog, XPLORE, uses subject terms supplied by the Library of Congress (video clip).