koine (n.)

The spoken LANGUAGE of a locality which has become a STANDARD language or LINGUA FRANCA. The term was originally used with reference to the Greek language used throughout the eastern Mediterranean countries during the Hellenistic and Roman periods; but it is now applied to cases where a VERNACULAR has come to be used throughout an area in which several languages or dialects are spoken, as in such notions as (for Old English) ‘West Saxon literary koine’ or (for US-influenced British English) ‘mid-Atlantic koine’.