Found on Wikipedia while verifying my use of the word "gloss" in my research project.
A gloss is a brief notation of the meaning of a word in a text. It may be in the language of the text, or in the reader's language if that is different.
A gloss was originally a note made in the margin or between the lines of a text in a classical language, in which the meaning of a word or passage is explained. As such, glosses can vary in thoroughness and complexity, from simple marginal notations of words one reader found difficult or obscure, to entire interlinear translations of the text with cross references to similar passages.
A collection of glosses is a glossary. A collection of medieval legal glosses, made by so called glossators, is called an apparatus. The compilation of glosses into glossaries was the beginning of lexicography, and the glossaries so compiled were in fact the first dictionaries. In modern times a glossary, as opposed to a dictionary, is typically found in a text as an appendix of specialized terms that the typical reader may find unfamiliar.
Link - http://en.wikipedia.org/wiki/Gloss