Rûm, also Roum or Rhum (in Arabic الرُّومُ ar-Rūm, Persian/Turkish Rum), is a very indefinite term used at different times in the Muslim world to refer to the Balkans and Anatolia generally, and for the Byzantine Empire in particular, for the Seljuk Sultanate of Rûm in Asia Minor, and for Greeks inhabiting Ottoman empire or modern Turkish territory as well as for Greek Cypriots. The name is a loan from the Byzantine-Greek self-designation Ρωμιοί "Romans". The city of Rome itself, by contrast, is known in Arabic as روما Rūmā.
The Qur'an includes Surat Ar-Rum (i.e., the Sura dealing with "The Romans" or "The Byzantines"). The Byzantine Greeks, as the continuation of the Roman Empire, called themselves Ρωμιοί or Ρωμαίοι Rhomaioi, Romans, and the Arabs, therefore, called them "the Rûm", their territory "the land of the Rûm", and the Mediterranean Sea "the Sea of the Rûm." They called ancient Greece by the name "Yūnān" (Ionia) and ancient Greeks "Yūnānī" (similar with Hebrew "Yavan" יוו for the country and "Yevanim" יוונים for the people). The ancient Romans were called either "Rūm" or sometimes "Latin'yun" (Latins).
Later, because Muslim contact with the Byzantine Greeks most often took place in Asia Minor, the term Rûm 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 Rûm, or the Sultanate of Rûm. But as the Mediterranean was "the Sea of the Rûm", so all peoples on its north coast were called sweepingly "the Rûm".
After the Fall of Constantinople, Mehmed II declared himself Kayser-i Rum, literally "Caesar of the Romans". However, later Ottoman Sultans abandoned this title and did not persist in claiming it. During the sixteenth century the Portuguese used "rume" and "rumes" (plural) as a generic term to refer to the Mamluk-Ottoman forces they faced then in the Indian Ocean.
Under the Ottoman Empire's Millet system, Greeks were in the "Rum Millet" (Millet-i Rum). The term "Urums", also derived from the same origin, is still used in contemporary ethnography to denote Turkic-speaking Greek populations. "Rumaiic" is a Greek dialect identified mainly with the Ottoman Greeks.
In Al-Andalus any Christian slave girl who had embraced Islam was named Roumiya. Also the legendary lover of King Roderic and daughter of Count Julian is named La Cava Rumía  — her affair being the putative cause of the Moorish invasion of Hispania in 711. 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 fourteenth century.
Al-Rūmī is a nisbah designating people originating in the Byzantine empire. Historical people so designated include:
- Suhayb ar-Rumi, a companion of Muhammad
- Mawlānā Jalāl-ad-Dīn Muhammad Balkhī (Rumi), the thirteenth century Persian poet
- Qāḍī Zāda al-Rūmī, fourteenth century mathematician
- Tadj ol-Molouk Ayrumlu, Former Queen of Iran
- W:Rûm Province, Ottoman Empire.
- W:Rumelia, from Turkish Rum eli meaning 'country of the Romans'.
- W:Erzurum, from the Turkish pronunciation of Arabic أرض روم arḍ Rūm, 'Land of the Romans'.
- Edirne Ciğeri, a Turkish meat dish also referred to as "Rumeli Ciğeri".
- W:Rumi calendar, a calendar based on the Julian Calendar, used by the Ottoman Empire after Tanzimat.
- W:Mevlana, great Persian poet who is sometimes referred to as Rumi.
- Rumiye-i Suğra, the name of the region in Ottoman Empire which included Tokat, Amasya, and Sivas.
- Rumçi, another term used to refer to the Greeks during the Ottoman times.
- Ozbaran, Salih, "Ottomans as 'Rumes' in Portuguese sources in the sixteenth century", Portuguese Studies, Annual, 2001
- Miguel de Cervantes, Don Quixote, Part I, Chapter 41 (Spanish text, English text).