In
sign language, a
gesture (or
combination of gestures) that expresses a particular
concept. For example, there are glosses for each
letter of
the alphabet, for the verb "
vomit", for the phrase "
as a matter of fact".
In learning a new language, we often encounter idioms that don't translate well. The English word "hangover", for example, is represented in German by "Katzenjammer" - figuratively, "cats running around". "Hangover" and "Katzenjammer" can be considered, respectively, the English and German glosses for the universally-understood concept of what one feels the morning after immoderate alcohol consumption.
In our ever-more-specialized world, we find that even an English-speaking biochemist and an English-speaking software engineer may have trouble understanding each others' professional jargon. So, we stick a glossary in the back of a book where new terms, and specialized uses of common terms, abound.