The glossary module
A glossary is a sort of dictionary that one usually considers as part of a book. Glossaries usually define special terms used within a book, to aid the reader to know what the author means. The Drupal Glossary module adds similar functionality to a Drupal website.
To use the Glossary module you create a vocabulary, e.g. Glossary. You add terms to that vocabulary, and the term description is to contain the definition for the glossary term. The glossary module generates pages of navigation to let users browse the glossary terms, and definitions. It also includes an input filter that modifies glossary terms appearing in any node to help the reader connect to the glossary term. It will show a tooltip describing the term, and will link to the glossary entry for this term.
Since the glossary uses the taxonomy system then glossary terms can only be added by an administrator with permissions to add vocabulary terms.
I haven't tried out the G2 Glossary module, but it is a different take on the same idea.