A gloss is a note made in the margins or between the lines of a book, in which the meaning of the text in its original language is explained in another language. 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 original text.

Glosses are of some importance in philology, especially if one language—usually, the language of the author of the gloss—has left few monuments of its own. The Reichenau glosses, for example, gloss the Latin Vulgate Bible in an early form of one of the Romance languages, and as such give insight into late Vulgar Latin at a time when that language was not often written down. A series of glosses in the Old English language to Latin Bibles give us a running translation of Biblical texts in that language; see Old English Bible translations. Glosses of Christian religious texts are also important for our knowledge of Old Irish. Glosses frequently shed valuable light on the vocabulary of otherwise little attested languages; they are less reliable for syntax, because many times the glosses follow the word order of the original text, and translate its idioms literally.

Glosses are extremely important in theology. They are a primary format in medieval Biblical theology, and were studied and memorized almost upon their own merit, without regards to the author. Many times a Biblical passage was heavily associated with a particular gloss, whose truth was taken for granted by many theologians.

A collection of glosses is a glossary. The compilation of glosses into glossaries was the beginning of lexicography, and the glossaries so compiled were in fact the first dictionaries.