(linguistics, sociology) The coexistence in a given population of two closely related native languages or dialects, one of which is regarded as more prestigious than the other; the similar coexistence of two unrelated languages.
2000, Joshua A. Fishman, Chapter 3: Bilingualism with and without diglossia; diglossia with and without bilingualism, Li Wei (editor), The Bilingualism Reader, Taylor & Francis (Routledge), page 81,
It is the purpose of this chapter to relate these two research traditions to each other by tracing the interaction between their two major constructs: bilingualism (on the part of psychologists) and diglossia (on the part of sociologists).
(pathology) The presence of a cleft or doubled tongue.