Medina: Also located in western Saudi Arabia, Medina is Islam's second-holiest place. Muhammad migrated to Medina with 70 Muslim families in 622 after being persecuted by the Meccan establishment. It is also the site of Muhammad's tomb.
Mosque: The Arabic word is masjid, meaning "place of prostration" before God. Muhammad built the first mosque in Medina. A mosque should be oriented toward Mecca. In many Islamic societies, mosques serve social and political functions in addition to religious ones.
Muslim: In Arabic, "one who surrenders to God"; a follower of Islam. There are 1 billion Muslims in the world and 6 million in the United States.
Shii: The "partisans" of Ali, the fourth caliph, the Shiis eventually became a distinct Muslim sect. The largest Shii Muslim sect is the "Twelver Shii," named after the first 12 leaders (or imams) of Shii Muslims. Twelver Shii believe that the descendants of Ali, Muhammad's cousin and son-in-law, were the legitimate leaders of Islam. Shiis believe the last imam is in hiding, and they await his return. Shiis are the majority in Iran, and many can be found in Iraq, Syria, Lebanon, and Pakistan. There are more than 165 million Shii Muslims in the world. (Also known as Shia or Shiite Muslims.)
Sunni: Unlike Shii Muslims, Sunni Muslims believe that Islamic leadership is vested in the consensus of the community, not in religious and political authorities. Their name comes from the word sunna, which is thought to mean "middle of the road." The religious scholar Karen Armstrong emphasizes that, despite their differences, Sunnis and Shiites alike observe the five pillars of Islam. "Like Judaism, Islam is a religion that requires people to live a certain way, rather than to accept certain credal propositions," she writes. "It stresses orthopraxy rather than orthodoxy."
Umma: The worldwide community of Muslims.
Wahabbism: A puritanical form of Islam that flourishes primarily in Saudi Arabia. It is named after Muhammad ibn al-Wahhab, an 18th-century Islamic reformer who wanted to return Islam to its beginnings by emphasizing a fundamentalist approach to the Koran.
Explainer thanks Karen Armstrong's A History of God and Islam: A Short History, Bernard Lewis' The Multiple Identities of the Middle East, and Encarta Online.