Sociolinguistics[r]: Branch of linguistics concerned with language in social contexts - how people use language, how it varies, how it contributes to users' sense of identity, etc. [e]
Language acquisition[r]: The study of how language comes to users of first and second languages. [e]
Lingua franca[r]: Any language used for widespread communication between groups who do not share a native language or where native speakers are typically in the minority; name from 'Lingua Franca', a pidgin once used around the Mediterranean. [e]
Contact language[r]: any language which is created through contact between two or more existing languages; may occur when people who share no native language need to communicate, or when a language of one group becomes used for wider communication. [e]
Language attrition[r]: The loss of a first or second language or a portion of that language by individuals. [e]
Diglossia[r]: Linguistic situation in which two (often very closely related) languages are used within one speech community, for different purposes. [e]