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.