A controlled vocabulary (CV) or managed vocabulary is an attempt to limit the number of terms that will be admitted into a discourse in order to improve communication.

In Content Management a CV may be used to remove ambiguities by substituting a Preferred Term for multiple equivalent synonyms available. The Preferred Term becomes the key label used in organizing the content navigation and a Variant Term, while deprecated, may be used in the Search Engine to improve Recall? and Precision?.

Authority File are lists of preferred terms to Use in a Controlled Vocabulary.

A Controlled Vocabulary organized hierarchically, with a broader term BT and a narrower term NT for each entry, becomes a Taxonomy.

When you add related terms RT, it becomes a Thesaurus.

References:
Information Architecture, Rosenfeld and Morville, pp. 177-188
Bob Boiko on Managing Vocabularies (Real Video file)
Controlled Vocabulary.com
IAWiki on CV
What Is A Controlled Vocabulary? (Boxes and Arrows)
CV Guide
A Thesaurus Project/Bibliography