Caliph is the term or title for the Islamic leader of the Ummah, or community of Islam. It is an Anglicized/Latinized version of the Arabic word خليفة or Khalīfah which means "successor", that is, successor to the prophet Muhammad. Some Orientalists wrote the title as Khalîf. The Caliph has often been referred to as Ameer al-Mumineen (أمير المؤمنين), or "Commander of the Faithful". The title has been defunct since 1924. Historically selected by committee, the holder of this title claims temporal and spiritual authority over all Muslims, but is not regarded as a possessor of a prophetic mission, as Muhammad is regarded in Islam as the final prophet.

Modern understandings of the title of Caliph are varied. Some movements in modern islamic philosophy have emphasized a protective dimension of Islamic leadership and social policy from an understanding of Khalifah that equates roughly to "render stewardship" or "protect the same things as God". Many Islamist movements have argued for the necessity of re-establishing the institution of a single office whose occupant, as successor to Muhammad, would possess clear political, military, and legal standing as the global leader of the Muslims. Such an initiative has yet to gather much in the way of practical support in the Muslim world.

The Sunnis identify the first four Caliphs, all close associates of Muhammad, as the '"rightly guided" caliphs: Abu Bakr, Umar Ibn Al-Khattab, Uthman Ibn Affan, and Ali Ibn Abu Talib.