A- The word " imam " in Arabic means "leader". In Islamic terminology it generally refers to any person who leads others in prayer. According to the early Sunni theologians, the Imam is the leader of the Islamic community, and his function is to enforce the revealed Law or Shari'ah . As such the term is equivalent to "caliph". In Sunnism it may also be an honorific term, given to certain important religious leaders, such as Imam Shafi'i, founder of one of the four Sunni schools of law. In Twelve-Imam Shi'ism it has two important meanings. As in Sunnism, the leader of others in prayer is called an " imam ", especially the person who performs this function on a regular basis in a mosque. But more specifically, an Imam is one of the twelve successors of the Prophet listed below.
The specific meaning given to the word "i mam " in Shi'ism can not be understood until one grasps the basic difference between the Sunni and Shi'ite branches of Islam. The roots of this difference are to be found in the differing views held by the companions of the Prophet concerning the nature of his successor or caliph. The Prophet himself performed three basic functions: He acted as the means whereby a celestial book, the Quran, was revealed by God to mankind. Thus he was the founder of a world religion. He was also the ruler of the early Islamic community, which means that he enforced the Shari'ah which God had revealed through the Quran. Finally he was the possessor of spiritual illumination and vision, and as such he could interpret the inner meaning of the Revelation and guide men upon the ascending stages of the path of spiritual perfection.
According to the majority of Muslims, the Sunnis, the successor of the Prophet must fulfill only one of these functions, i.e., he should enforce the Shari'ah . Muhammad had been the last Prophet, so there could be no prophet after him. And there was no way the community could guarantee that his successors would possess spiritual vision and illumination, for like prophecy, these things are divinely bestowed (although unlike prophecy, they could still be possessed by men). But undoubtedly, the Prophet's successor could act as a ruler and enforce the Shari'ah . In fact, the earthly existence of Islam largely depended upon this function being fulfilled, particularly at its beginning. Finally, the Sunnis held that the Prophet had not appointed a successor during his lifetime, so it was up to them to choose one.
But the minority group, known as the "Shi'ites" (the "partisans" of 'Ali), maintained that the Prophet's successor must not only enforce the Shari'ah , he must also possess divinely illuminated wisdom and be the spiritual guide of men. Since this latter function is bestowed by God and cannot be judged by the majority of men, the Prophet's successor must be divinely appointed, as expressed in the Prophet's wishes. And the Shi'ites hold that the Prophet had in fact appointed 'Ali as his caliph.
This difference in view between the Shi'ites and Sunnis was often expressed in political terms, resulting in a good deal of strife in the early centuries of Islam between certain Shi'ite groups and the Umayyad and Abbasid caliphs. For, as far as the Shi'ites were concerned, the Imams were the only completely legitimate successors to the Prophet. The first, 'Ali, was appointed by the Prophet himself, and each in turn was appointed by his predecessor according to divine decree.
B- Leader of mosque prayers: a man who leads the prayers in a mosque. One who leads the congregation prayers in a mosque, in a masjid, etc.