More general terms:
* Bilingualism: This is the most common and general term, but it doesn't capture the specific power dynamics and usage domains of diglossia.
* Language duality: This term is more specific than "bilingualism" but still doesn't fully capture the concept of diglossia.
* Code-switching: This refers to the act of switching between languages, which is a component of diglossia but not the whole phenomenon.
More specific terms:
* Polyglossia: This refers to the use of multiple languages in a society, which is similar to diglossia but can encompass more complex situations.
* Language variation: This is a broader term that refers to any differences in language use, but can be used to refer to the different varieties used in diglossia.
Alternative expressions:
* "High" and "low" language: This is a common way to refer to the two languages involved in diglossia.
* Formal and informal language: This is another common way to describe the different domains of use for the two languages.
Ultimately, the best synonym for "diglossia" will depend on the specific context in which you are using the word.