lingua franca

In the Middle Ages, the Arabs of the eastern Mediterranean referred to all Europeans as Franks (the name of the tribe that once occupied the land we call France). Since there was plenty of Arab-European trade, the traders in the Mediterranean ports eventually developed a trading language combining Italian, Arabic, and other languages, which almost everyone could more or less understand, and it became known as the "Frankish language", or lingua franca. Some languages actually succeed in becoming lingua francas without changing much. So, when the Roman empire became vast and mighty, Latin became the important lingua franca; and at a meeting between Japanese and Vietnamese businesspeople today, English may well be the only language spoken.