a situation in which two languages (or two varieties of the same language) are used under different conditions within a community, often by the same speakers. The term is usually applied to languages with distinct ‘high’ and ‘low’ (colloquial) varieties, such as Arabic.
1950s: from Greek diglōssos 'bilingual', on the pattern of French diglossie
diglossia in other Oxford dictionaries
Definition of diglossia in the US English dictionary