Gloss: (n) [alter. of gloze, fr. ME glose, fr. AF, fr. ML glosa, glossa, fr. Gk glossa, glotta, meaning tongue, language, obscure word]: a brief explanation, as in the margin or between lines, of a difficult or obscure word or expression. Gloss is also a transitive verb meaning to write a gloss.
Glossary: a collection of glosses. The adjective is glossarial.
Glossator: one that makes textual glosses; a compiler of a glossary; also called a glossarist or glossographer.
Glossography (n): The writing or compilation of glosses.
Definitions excerpted from Merriam-Webster.