The Library of Congress Subject Headings and the Subject Terms in databases are examples of "controlled vocabulary."
Controlled vocabulary terms are selected from the pre-existing list of LC subject headings and are assigned to the work by the cataloger based upon what the book, article, etc., is actually about. A keyword search is simply a matching exercise - the computer finds the words used by the searcher within the database or catalog. However, the computer has no understanding of what the user is looking for or the context of the keywords used. By contrast, when the authorized subject terms are used, the results will include all materials in that database that are about that particular subject. A human being (a librarian) has looked at the source and has determined what the source is about. Controlled vocabulary/subject headings searches are less flexible in terms of selecting words used in the search, but the results are more accurate and efficient.
Use the "Thesaurus" or "Subject Terms" list to find the authorized subject headings used by the database.
Examples in Academic Search Complete: senior citizens - older people
hacking - computer hacking
dual enrollment - dual school enrollment
