As a noun: explanatory notes or comments written on a text. As a verb: to write explanatory notes, or to apply a shiny finish.
From Old Norse 'glossa' (explanation). The verb form means to annotate or shine, while the noun refers to the annotations themselves or shiny surfaces.
The same word 'gloss' describes both medieval manuscript margins and today's cosmetics—both are ways of adding attractiveness or clarity to something existing!
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