Diglossia is a linguistic term that is also used in sociology. Diglossia is the coexistence of language codes that are either (1) two varieties of one language or (2) two distinct languages. Diglossic speakers use both codes in culturally defined situations in which the situation excludes the use of one diglossic code while requiring the use of the other. Marie-Christine Hazaël-Massieux gives a precisely detailed definition of diglossia in the excerpt from "Diglossia" quoted above.
What this means is that there are cultures, from England to Switzerland to South Africa to the United States, in which two competing language systems (i.e. codes) govern competing cultural situations with one code governing particular situations at the exclusion of the other. In Switzerland, for example, university professors may discuss academic issues amongst themselves in their own regional dialect or in Low French or Low German, yet give course lectures in High French or High German. Each of these would be a case of two varieties of the same language in competition: the formality of lectures excludes the option of a dialectical variety or a Low variety of the language, while a casual situation amongst peers excludes the necessity of a High variety.
In South Africa, domestic servants will speak their own language, perhaps Xhosa, at home or when conducting commerce amongst themselves while changing to English or Afrikaans for communication on their jobs. More interestingly, Afrikaners and English speakers may often be heard conducting a single conversation in both Afrikaans and English: the English speaker speaks in English while the Afrikaans speaker responds in Afrikaans to which the English speaker responds in English, and on and on throughout the whole conversation. Both these examples represent diglossic situations between two languages: each language has a culturally defined realm of required or appropriate use, and sometimes even these defined situations contain diglossic competition within.
Other examples of two varieties in competition in diglossic situations are:
- Canadian French and High French.
- United States African American Vernacular English and Standard American English.
- Haitian French creole and French.
Another example of two languages in diglossic competition is in India where Hindi may be used at home as the heritage language and Indian English (a variety of British English) is used daily for communication, business, commerce, entertainment, and education.