Diglossia is a linguistic phenomenon found in many multilingual speech communities. Diglossia describes a particular type of sociolinguistic situation in which there is a clear differentiation in function between the languages or language varieties used in a bilingual/multilingual community. One linguistic variety, the high variety (H-variety), is the prestige variety - generally a standard variety - and is typically reserved for official functions in more formal speech situations in the public sphere (e.g. in government, in written education, for religious service, or by the media). The other linguistic variety, the low variety (L-variety), is exclusively used in and restricted to informal speech situations in the private sphere. The L-variety usually has less prestige than the H-variety and is made of the vernacular varieties used at home for informal everyday conversations. This specialization of function between H and L is seen as the most important criterion for the classification of a speech community as diglossic. While H is appropriate only in formal situations, L is only used in informal situations.

H and L differ from each other both linguistically and socially. Linguistically, they do so with respect to grammar, phonology, and vocabulary; socially, they differ in function and prestige, as well as in literary heritage, acquisition, standardization, and stability.

Ferguson's (1959) Concept of Diglossia

Fishman's (1967) Concept of Diglossia