A glossary is an alphabetically arranged list of terms, with a definition or an explanation of each term. A term can be a single word or many words. Typically, in a printed document, the glossary is at the end of the document. Usually, in online help, each term in a topic, or the first instance of a term, has a popup that explains the term.
Unwalla, Mike. TechScribe (2007). Articles>Documentation>Technical Writing>Glossary