Caliph: A religious leader in Islam, believed to be the successor to the Prophet Muhammad. The caliph is the head of the ummah, or the community of the faithful. Over time, the caliphate became a religio-political position, in which the caliph ruled over the Muslim empire.
Brief History of the Caliphate:
The original schism between Sunni and Shi'a Muslims occured after the Prophet died because of a disagreement over who should be the caliph. Those who became the Sunnis believed that any worthy follower of Muhammad could be caliph, and they backed the candidacies of Muhammad's companion, Abu Bakr, and then of Umar when Abu Bakr died. The early Shi'a, on the other hand, believed that the caliph should be a close relative of Muhammad. They preferred the Prophet's son-in-law and cousin, Ali.
After Ali was assassinated, his rival Mu-waiyah established the Umayyad Caliphate in Damascus, which went on to conquer an empire stretching from Spain and Portugal in the west through North Africa and the Middle East to Central Asia in the east. The Umayyads ruled from 661 to 750 CE, when they were overthrown by the Abbasid Caliphs.
From their capital at Baghdad, the Abbasid caliphs ruled from 750 to 1258, when the Mongol Hulagu Khan sacked Baghdad and executed the caliph. In 1261, the Abbasids regrouped in Egypt, continuing to exert religious authority until 1519, when the caliphate moved to Constantinople, the capital of the Ottoman Empire.
The caliphs continued as heads of the Muslim world (though not universally recognized as such, of course) until Mustafa Kemal Ataturk abolished the caliphate in 1924. Although this move by the newly secular Republic of Turkey sparked outcry among other Muslims around the world, no new caliphate has ever been recognized.
The word "caliph" comes from the Arabic khalifa, meaning "substitute" or "successor." In this usage, khalifa is closer in meaning to "representative"; the caliphs represent Muhammad during their time on earth.