The term caliphate, "dominion of a caliph ('successor')" (from the Arabic خلافة or khilāfa, Turkish: Hilafet), refers to the first system of government established in Islam and represented the leaders unity of the Muslim Ummah (community)^{[clarification needed]}. In theory, it is an aristocratic–constitutional republic (the Constitution being the Constitution of Medina), which means that the head of state, the Caliph, and other officials are representatives of the people and of Islam and must govern according to constitutional and religious law, or Sharia. In its early days, it resembled elements of direct democracy (see shura) and an elective monarchy.

It was initially led by Muhammad's disciples as a continuation of the leaders and religious system the prophet established, known as the 'Rashidun caliphates'. A "caliphate" is also a state which implements such a governmental system.

Sunni Islam stipulates that the head of state, the caliph, should be selected by Shura – elected by Muslims or their representatives. Followers of Shia Islam believe the caliph should be an imam chosen by God from the Ahl al-Bayt (Muhammad's purified progeny). After the Rashidun period until 1924, caliphates, sometimes two at a single time, real and illusory, were ruled by dynasties. The first dynasty was the Umayyad. This was followed by the Abbasid, the Fatimid (not recognized by others outside Fatimid domain), and finally the Ottoman Dynasty.