Definition -
(1) An unusual or obscure word that needs explanation.
(2) A note made in a margin to explain an obscure word.

Etymology -
The word derives via Latin from the Greek glossa (Ionic), obscure word (literally: tongue).

Oxford English Dictionary -
Its first citation for sense (2) is from 1548:
"Like as by a glosse ye subuerte the commaundement."
( Udall, etc. Erasm. Par. Matt. xxiii. 108)

Quotation -
"Such glosses have played an important role in the history of lexicography. Old English marginal or interlinear glosses of hard Latin words were collected in more or less alphabetical lists, known as glossae collectae (collected glosses) then glossaria (glossaries). These were ancestors of the first Latin-English dictionaries and ultimately of all English dictionaries."
(Source: Oxford Companion to the English Language s.v. gloss )

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