Definition - A language that has two levels:
(1) a prestigious level that is used in law and government (e.g. Classical Latin) and
(2) a more colloquial, less prestigious level that is the language of daily discourse (e.g. Vulgar Latin).

Etymology -
The term was introduced to English by C. A. Ferguson in 1959. He derived it from the French diglossie, the existing French term for the phenomenon; the latter was coined by the Greek linguist Ioannis Psycharis, who coined it as a transliteration of the Greek diglossia, bilingualism.

Oxford English Dictionary -
Its first citation is from 1959:
"In many speech communities two or more varieties of the same language are used by some speakers under different conditions.
Ibid., The term ‘diglossia’ is introduced here, modeled on the French diglossie, … since there seems to be no word in regular use for this in English."
(C. A. Ferguson in Word XV. 325 Diglossia.)

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