A gloss 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. 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. Also, satirical explanations of words and events are called glosses. The German Romantic movement used the expression of gloss for poems commenting on a given other piece of poetry, often in the Spanish Décima style.

Glosses were originally notes made in the margin or between the lines of a text in a classical language; the meaning of a word or passage is explained by the gloss. As such, glosses vary in thoroughness and complexity, from simple marginal notations of words one reader found difficult or obscure, to interlinear translations of a text with cross references to similar passages. Today parenthetical explanations in scientific writing and technical writing are also often called glosses. Hyperlinks to a glossary sometimes supersede them.