Diglossia
Diglossia is a linguistic situation in which two (often very closely related) languages are used within one speech community, or when a language has two standard forms. This often means that one of the language forms is used as the colloquial language in everyday life, while the other is mainly used in its written form and thereby often has more prestige as well.
A well-known example of a language used in a diglossia situation is the Norwegian language. It is divided into Bokmål and Nynorsk, the former of which being more or less archaic and used mainly in formal situations, while the latter is used in everyday life and has a lot in common with the popular dialects. Also Arabic is an example of a diglossic language'it has two standarized forms, one of which being very archaic and only used in its written form, while the other is the vernacular. The difference between these two forms of Arabic is very big.
Situations of diglossia are especially interesting for sociolinguists, who study the different ways a language is used in diverging situations.