Section 1
What is a Thesaurus?

A thesaurus is a tool for vocabulary control. By guiding indexers and searchers about which terms to use, it can help to improve the quality of retrieval.

Usually, a thesaurus is designed for indexing and searching in a specific subject area. Examples of subject areas covered by thesauri are education, metallurgy, and art and architecture.

What is in a thesaurus?

A thesaurus gives several types of information to indexers and searchers.

Preferred Terms

Obviously, the thesaurus has to indicate which terms indexers and searchers are allowed to use. These terms are called preferred terms.

This is a major part of vocabulary control - restricting the vocabulary so that it is easier to predict what words might have been used to index a concept.

Non-preferred Terms

In addition to preferred terms, a thesaurus also needs to indicate some terms that indexers and searchers are not to use.

These terms are called non-preferred terms.

It should be possible to look up a non-preferred term and see what preferred term should be used instead. This will save time and make it less likely that the best preferred term will be missed.

A thesaurus also usually allows you to look up a preferred term and see its non-preferred terms. This can give you a better idea of what the term is supposed to mean.

Semantic Relations

As well as linking preferred terms with non-preferred terms, a thesaurus also shows links between different preferred terms.

These links are usually for semantic relations.

Like a link between a preferred term and a non-preferred term, one of these semantic links can help to direct you to the right term and make the meaning of a term clearer.

Guides to Application

A good thesaurus should make it clear what a term is meant to cover. It can accomplish this to some extent by showing non-preferred terms and semantic relations. Other ways of guiding people in using a thesaurus include introductory matter and scope notes.

A scope note often takes the form of a definition of the term.

Ensuring that terms are used consistently with the same meaning is another major aspect of vocabulary control.

Rules for Synthesis

Usually, a thesaurus lists all its preferred terms explicitly.

Such thesauri are enumerative.

Some thesauri indicate some preferred terms indirectly: instead of listing all the preferred terms, they give rules for creating them out of components.

Such thesauri are at least partly synthetic.


You would expect the following topics to be covered in a thesaurus:
  1. preferred terms
  2. non-preferred terms
  3. semantic relations between terms,
  4. how to apply terms
    and sometimes
  5. how to produce terms that are not listed explicitly in the thesaurus.
quiz Quiz on thesaurus content (requires JavaScript)

Introduction Section 2 Table of Contents Glossary
Last updated January 25, 2008, by Tim Craven
Copyright © 1997 The University of Western Ontario