imam

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun In law and theology, the caliph who is successor to Muhammad as the lawful temporal leader of the Islamic community.
• noun The male prayer leader in a mosque.
• noun The Muslim worshiper who leads the recitation of prayer when two or more worshipers are present.
• noun In Twelver Shia belief, any of 12 descendants of Muhammad regarded as divinely appointed spiritual and temporal leaders.
• noun A ruler claiming descent from Muhammad and exercising authority in an Islamic state.
• noun Any of the founders of the four schools of law and theology.
• noun An authoritative scholar who founds a school of law or theology.
• noun Used as a title for an imam.

from The Century Dictionary.

• noun A Mohammedan chief or leader. Specifically

from the GNU version of the Collaborative International Dictionary of English.

• noun Among the Mohammedans, a minister or priest who performs the regular service of the mosque.
• noun A Mohammedan prince who, as a successor of Mohammed, unites in his person supreme spiritual and temporal power.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A Shi'ite Muslim leader.
• noun One who leads the salat prayers in a mosque.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (Islam) the man who leads prayers in a mosque; for Shiites an imam is a recognized authority on Islamic theology and law and a spiritual guide