A gloss (from Latin: glossa; from Greek: γλῶσσᾰ glôssa, meaning ‘language’) is a brief notation, especially a marginal one or an interlinear one, of the meaning of a word or wording in a text. It may be in the language of the text, or in the reader’s language if that is different.
A collection of glosses is a glossary. A collection of medieval legal glosses, made by 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.
Source: Gloss (annotation) – Wikipedia