Boxes and Arrows has a new article on A Controlled Vocabulary:

A controlled vocabulary is a way to insert an interpretive layer of semantics between the term entered by the user and the underlying database to better represent the original intention of the terms of the user.

Using a controlled vocabulary can make searches more meaningful by interpreting what a user enters so that it makes sense for the underlying database. For example, 'dress pants' and 'trousers' might really be the same thing, but why make users guess which one you used?