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.
Starting in the 14th century, a gloze in the English language was a marginal note or explanation, borrowed from French glose, which comes from medieval Latin glōsa, classical glōssa, meaning an obsolete or foreign word that needs explanation. Later, it came to mean the explanation itself. The Latin word comes from Greek γλῶσσα 'tongue, language, obsolete or foreign word'. In the 16th century, the spelling was refashioned as gloss to reflect the original Greek form more closely.
Biblical gloss
Glosses and other marginal notes were a primary format used in medieval Biblical theology and were studied and memorized for their own merit.