Diglossia

Etymology

From Greek "di-" (two) and "glōssa" (tongue, language), via French "diglossie." The concept was given its technical definition by the linguist Charles Ferguson in 1959, who distinguished it from ordinary bilingualism.

noun · /dy-GLOSS-ee-uh/
A situation in which two forms of a language are used by one community for sharply different settings — one formal, one everyday.
What it names
A common but easily missed social pattern: a "high" variety for writing and ceremony and a "low" one for home, each with its own turf.
Example
Arabic-speaking countries live in diglossia: a shared formal standard for news and books, and distinct spoken dialects for the street.
Related idea
Diglossia is two varieties of one language split by function within a community; bilingualism is command of two separate languages by an individual — the first is a social division of labor, the second a personal skill.