Etymologically diglossia means ‘two languages’, but this term is used in the literature to denote a specific type of social bilingualism and is particularly helpful as it allows us to better understand a number of sociolinguistic scenarios. Diglossia is defined by Sayahi as ‘a situation where two linguistic systems coexist in a functional distribution within the same speech community’ (2014: 1). Within some diglossic communities, certain varieties are tied to issues of prestige, with typically one being the literary and prestigious form (or High), and the other(s) oral and popular (or Low) (see Fishman 1967: 31). This occurs, for example, in Greece wherein Katharevusa is the High variety, heavily influenced by the historical and literary prestige of Ancient Greek, and Demotic is the Low oral vernacular.
Fishman, J., (1967) ‘Bilingualism With and Without Diglossia; Diglossia With and Without Bilingualism’. Journal of Social Issues 23 (2): pp. 29-38.
Sayahi, L., (2014) Diglossia and Language Contact: Language Variation and Change in North Africa. Cambridge University Press: Cambridge.