(“leader,” “pattern”), the head of the Muslim community; the title is used in the Qurʾān several times to refer to leaders and to Abraham. The origin and basis of the office of imam was conceived differently by various sections of the Muslim community, this difference providing part of the political and religious basis for the split into Sunnite and Shīʿite Islām. Among Sunnites, imam was synonymous with caliph (khalīfah), designating the successor of Muḥammad, who assumed his administrative and political, but not religious, functions. He was appointed by men and, although liable to error, was to be obeyed even though he personally sinned, provided he maintained the ordinances of Islām.

Political disagreement over succession to his office after the death (661) of Alīʿ, the fourth caliph and Muḥammad's son-in-law, propelled the Shīʿite imam along a separate course of development, as partisans of ʿAlī attempted to preserve leadership of the entire Muslim community among the descendants of ʿAlī. In Shīʿite Islām, the imam became a figure of absolute spiritual authority and fundamental importance. ʿAlī and the successive imams, who are believed by Shīʿism to be the sole possessors of secret insights into the Qurʾān given them by Muḥammad, under Neoplatonic influences of the 9th–10th centuries AD became viewed as men illumined by the Primeval Light, God, and as divinely appointed and preserved from sin. They alone, and not the general consensus of the community (ijmāʿ) essential to Sunnite Islām, determined matters of doctrinal importance and interpreted revelation. With the historical disappearance (ghaybah) of the last imam—there has been no consistency in the number legitimized: among the major sects, Sabʿīyah Ismāʿīlīs acknowledge 7 imams and Ithnā ʿAsharī (Twelvers) 12—there arose a belief in the hidden imam, who is identified with the mahdī.

Imam has also been used as an honorary title, applied to such figures as the caliph ʿAlī ibn Abī Ṭālib and the theologians Abū Ḥanīfah, ash-Shāfiʿī, Mālik ibn Anas, Aḥmad ibn Ḥanbal, al-Ghazālī, and Muḥammad ʿAbduh. The title imam also is sometimes given to the specially trained Muslims who lead prayers in the mosques.