Imam is an Arabic word meaning "Leader". The ruler of a country might be called the Imam, for example. The term, however, has important connotations in the Islamic tradition.
The common everyday use of the word is for a person leading Muslim congregational prayers. In this meaning Imam is not required to be a cleric.
The term is also used for a recognized religious leader or teacher in Islam, often for the founding scholars of the four Sunni madhhabs, or schools of religious jurisprudence (fiqh). For example, Imam Abu Hanifa. Or the Imams of the sciences related to Hadith such as Imam Bukhari, Imam Muslim, Imam Abu Dawood. Or the heads of the Prophet's decendants in their times such as Imam Jafar Sadiq.
In the Shia context, Imam also has a meaning more central to belief.
The Shia interpretation is that the Quran clearly says that only God can appoint an Imam and no one else has the power to designate one. The incident of Ghadeer-e-Khum is referenced as when Muhammad declared Ali as the leader of the community after him.
According to the Twelve-Imam Shiite dogma, imam is a divine status, greater than the status of a Prophet. According to this ideology this title is reserved for only twelve early leaders of Islam—Ali, Hasan, Husayn, and nine of Husayn's descendants.
Following is a listing of the rightful sucessors of Muhammad, as recognized by mainstream ("Twelver") Shias. Each Imam was the son of the previous Imam, except for Husayn who was the brother of Hasan. See Shia Imams for details.
- Ali ibn Abu Talib (600 - 661)
- Hasan ibn Ali (625 - 669)
- Husayn ibn Ali (626 - 680)
- Ali ibn Husayn (658 - 713), also known as Zainul Abideen
- Muhammad al Baqir (676 - 743)
- Jafar as Sadiq (703 - 765)
- Musa al Kazim (745 - 799)
- Ali ar Rida (765 - 818)
- Muhammad at Taqi (810 - 835)
- Ali al Hadi (827 - 868)
- Hasan al Askari (846 - 874)
- Muhammad al Mahdi (868 - ?))
The Ismailis trace a different line of Imams, branching at one of Husayn's descendants, Ismail bin Jafar.
The Zayidis trace a different line, branching at Husayn's grandson, Zaid.
- For a slightly more detailed description of the Shiite belief (and for the other names and titles of the twelve Imáms), see, for example, http://www.bahai-library.org/books/dawnbreakers/preface/prefislam.html .
- Graphical illustration of the Shia sects (http://philtar.ucsm.ac.uk/encyclopedia/islam/shia/index.html)
- Akhbari sub-sect (http://www.akhbari.com), a Shia school that rejects modern innovations; including Khomeini's