Ali (Ali ibn Abu Talib), 598?-661, 4th caliph (656-61). The debate over his right to the caliphate caused a major split in Islam into Sunni and Shiite branches, and he is regarded by the Shiites as the first Imam, or leader: Shiite derives from the phrase shi-at Ali [Ar.=the party of Ali]. He was the son of Abu Talib, Muhammad's uncle, but was more closely related to the Prophet as the husband of Fatima. He was one of the Prophet's first and most faithful followers. There are conflicting traditions on whether Muhammad intended Ali to be his immediate sucessor. In 656 he became the 4th caliph on Uthman's death. He was strongly opposed by Aishah, the Prophet's wife; Muawiya, who later founded the Umayyad dynasty; and dissatisfied ex-supporters, the Khawarijis. The Khawarijis succeeded in assassinating him in 661. His eldest son Hasan (624-70) abdicated in favor of Muawiya. Ali and his second son Husein (626-80) are the great martyrs of the Shiites.