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Shia Islam (/ ˈ ʃ iː ə /) or Shi'a Islam is the second-largest branch of Islam. It holds that the Islamic prophet Muhammad designated Ali ibn Abi Talib (656–661 CE) as his successor (Arabic: خليفة, romanized: khalifa) and the Imam (Arabic: امام, lit. ' spiritual and political leader ') after him, most notably at the event of Ghadir Khumm, but was prevented from succeeding Muhammad as the leader of the Muslims as a result of the choice made by some of Muhammad's other companions (Arabic: صحابہ, romanized: sahaba) at Saqifah. This view primarily contrasts with that of Sunni Islam, whose adherents believe that Muhammad did not appoint a successor before his death and consider Abu Bakr, who was appointed caliph by a group of Muhammad's other companions at Saqifah, to be the first rightful (rashidun) caliph after Muhammad (632–634 CE). Adherents of Shia Islam are called Shia Muslims.

Shia Muslims' belief that 'Ali was the designated successor to Muhammad as Islam's spiritual and political leader later developed into the concept of Imamah, the idea that certain descendants of Muhammad, the Ahl al-Bayt (Arabic: أَهْل ٱلْبَيْت, lit. ' People of the House '), are rightful rulers or Imams through the bloodline of 'Ali and his two sons Hasan and Husayn, whom Shia Muslims believe possess special spiritual and political authority over the Muslim community. Later events such as Husayn ibn Ali's martyrdom in the Battle of Karbala (680 CE) further influenced the development of Shia Islam, contributing to the formation of a distinct religious sect with its own rituals and shared collective memory.

Shia Islam is followed by 10–15% of all Muslims. Although there are many Shia subsects in the Muslim world, Twelver Shi'ism is by far the largest and most influential, comprising about 85% of all Shia Muslims. Others include the Isma'ili, Zaydi, and Alevi. Shia Muslims form a majority of the population in four countries across the Muslim world: Iran, Iraq, Azerbaijan, and Bahrain. Significant Shiite communities are also found in Lebanon, Kuwait, Turkey, Yemen, and the Indian subcontinent. Iran stands as the world's only country where Shia Islam forms the foundation of both its laws and governance system.