In linguistics, diglossia (/dˈɡlɒsiə, dˈɡlɔːsiə/) is a situation in which two dialects or languages are used (in fairly strict compartmentalization) by a single language community. In addition to the community's everyday or vernacular language variety (labeled "L" or "low" variety), a second, highly codified lect (labeled "H" or "high") is used in certain situations such as literature, formal education, or other specific settings, but not used normally for ordinary conversation.[4] In most cases, the H variety has no native speakers but various degrees of fluency of the low speakers.

The station board of Hapur Junction railway station in Northern India; Hindustani is an example of triglossia, with a common vernacular and two formal registers.[1] Furthermore, digraphia is present between the two formal registers.[2][3]

The high variety may be an older stage of the same language (as in medieval Europe, where Latin remained in formal use even as colloquial speech diverged), an unrelated language, or a distinct yet closely related present-day dialect, for example Hindustani (L) alongside the standard registers of Hindi (H) and Urdu (H); or Modern Standard Arabic alongside other varieties of Arabic; or Chinese, with Mandarin as the official, literary standard and local varieties of Chinese used in everyday communication.[1][5] Other examples include literary Katharevousa versus spoken Demotic Greek; Indonesian, with its Baku and Gaul forms;[6] and literary versus spoken Welsh.

Garifuna (Karif) of Central America is unusual in that it has gender-based diglossia men and women quite often have different words for the same concepts.