koine (n.)

common literary dialect of Greek in the Roman and early medieval period, 1903, from feminine singular of Greek koinos "common, ordinary" (see coeno-). Used earlier as a Greek word in English. From 1926 of other dialects in similar general use.

[Isocrates] helped to lay the foundations for that invaluable vehicle of civilization, the Koinê Dialektos, through which, at the price of becoming easy, flat, common, and a little soulless, the Greek language in the Hellenistic period evangelized the whole Mediterranean world. [Gilbert Murray, "Greek Studies," 1946]