imam (ĭmämˈ) [key] [Arab., = leader], in Islam, a recognized leader or a religious teacher. Among the Sunni the term refers to the leader in the Friday prayer at the mosque; any pious Muslim may function as imam. The term has also been used as a synonym for caliph (see caliphate), the vicegerent of God. The Shiites, with their numerous denominations throughout history, have developed specific meanings for the term. Zaydi Shiites recognize as Imam any pious descendant of Ali and Fatima who earns his recognition as a leader through struggle. Twelve-Imam Shiite dogma restricts the Imams to Ali, his sons Hasan and Husein, and nine direct linear descendants of Husein. Twelve-Imam Shiite doctrine presents the Imams as infallible intermediaries between the human and the divine. The continuous presence of the Imams being a prerequisite for human salvation, al-Mahdi, the last Imam, is considered in occultation (hidden from humanity) since 874 only to return near the end of creation as a messiahlike figure. For Ismaili Shiites, the succession of the Imams breaks off from the Twelve-Imams line with Ismail, the son of Jafar al-Sadiq (see Ismailis). At present the Nizari subgroup of the Ismailis is the only group whose members claim a living and visible Imam in the person of Prince Karim al-Hussayni, Aga Khan IV (see Aga Khan). The use of the title imam by the Iranian revolutionary leader Ruhollah Khomeini and by the Lebanese Shiite leader Musa al-Sadr signaled a new development in Twelve-Imam Shiite doctrine, since neither could not claim to be the Hidden Imam returned, reflecting the desire to transcend the passive waiting for the reappearance of the Mahdi and promote the reincorporation of political activism into Shiite religious life.