Rüm, also Roum or Rhum (in Arabic ar-Rum), is a very indefinite term used at different times in the Islamic world for Europeans generally and for the Byzantine Empire in particular, for the Seljuk Sultanate of Rüm in Asia Minor, and for Greeks inhabiting Ottoman territory.
When the Arabs met the Byzantine Greeks, these called themselves Rhomaioi, or Romans, and the Arabs, therefore, called them "the Rüm" as a race-name, their territory "the land of the Rüm", and the Mediterranean "the Sea of the Rüm." The original ancient Greeks they called "Yünãn" (Ionians), the ancient Romans, "Rüm" and sometimes "Latin'yun" (Latins). Later, because Muslim contact with the Byzantine Greeks most often took place in Asia Minor, the term Rüm became fixed there geographically and remained even after the conquest by the Seljuk Turks, so that their territory was called the land of the Seljuks of Rüm, or the Sultanate of Rüm. But as the Mediterranean was "the Sea of the Rüm", so all peoples on its north coast were called sweepingly "the Rüm". In Spain any Christian slave-girl who had embraced Islam was named Rumiya, and we find the crew of a Genoese vessel being called Romans by a Muslim traveller. The crusades introduced the Franks (Ifranja), and later Arabic writers recognize them and their civilization on the north shore of the Mediterranean west from Rome; so Ibn Khaldun wrote in the latter part of the 14th century. Even in the modern-day Maghreb, any Westerner is liable to be called Rumi in the countryside at least, and the French language is often called ar-Rumiyya.