(“successor”), ruler of the Muslim community. When Muḥammad died (June 8, 632), Abū Bakr succeeded to his political and administrative functions as khalīfah rasūl Allāh, or “successor of the Messenger of God,” but it was probably under ʿUmar ibn (Umar Iʿ) al-Khaṭṭāb, the second caliph, that the term caliph came into use as a title of the civil and religious head of the Muslim state. In the same sense, the term was employed in the Qurʾān in reference both to Adam and to David as the vice-regents of God.

Abū Bakr and his three immediate successors are known as the “perfect” or “rightly guided” caliphs (al-khulafāʾ ar-rāshidun). After them the title was borne by the 14 Umayyad (Umayyad Dynasty) caliphs of Damascus and subsequently by the 38 ʿAbbāsid (Abbāsid Dynastyʿ) caliphs of Baghdad, whose dynasty fell before the Mongols in 1258. There were titular caliphs of ʿAbbāsid descent in Cairo under the Mamlūks from 1258 until 1517, when the last caliph was captured by the Ottoman (Ottoman Empire) sultan Selim I. The Ottoman sultans then claimed the title and used it until it was abolished by the Turkish Republic on March 3, 1924.

After the fall of the Umayyad dynasty at Damascus (750), the title of caliph was also assumed by the Spanish branch of the family who ruled in Spain at Córdoba (755–1031), and it was also assumed by the Fāṭimid (Fāṭimid Dynasty) rulers of Egypt (909–1171), who claimed to descend from Fāṭimah (daughter of Muḥammad) and her husband, ʿAli.

According to the Shīʿite Muslims, who call the supreme office the “imamate,” or leadership, no caliph is legitimate unless he is a lineal descendant of the Prophet Muḥammad. The Sunnites insist that the office belongs to the tribe of Quraysh (Koreish), to which Muḥammad himself belonged, but this condition would have vitiated the claim of the Turkish sultans, who held the office after the last ʿAbbāsid caliph of Cairo transferred it to Selim I.