A controlled vocabulary is a set of agreed-upon words or phrases that are used to describe the subjects of the articles or books in a database or online catalog system. Each book or article listed in a database or online catalog system has assigned to it one or more of these words or phrases. These usually appear somewhere in the record in a "Subject" or "Descriptor" field.
Example: [From Albert, the Saint Albert Hall Library online catalog]
Why use a controlled vocabulary?
It is often difficult to imagine all of the possible words that might be used to describe a particular subject or concept. The reality is that different writers may use different words to describe the subject of a book. A controlled vocabulary can help us to deal with this problem.
If we are using an online catalog or database, and we know that the subject heading or descriptor for the term is AIDS, then we can do an Exact Subject search on AIDS and retrieve all items pertaining to AIDS even when the titles of some of the books used Acquired Immune Deficiency Syndrome.