Since the terrorist atrocities of September 11, 2001, Westerners have been challenged to understand the ideological and theological concepts, derived from Islam, that motivated the actions of Al-Qaida on that day and in other attacks before and since. Differences in taxonomy have proven to be a major issue. In the author's view, it is insufficient to assign to the terrorists an "Islamist" intent. Although "Islamism" and "political Islam" are widely used to designate radicalism in the religion, neither has been provided by scholars with a precise meaning. In this article, the author untangles the confusing terminology used to describe radical Islam today. Most important, he clarifies the history and differences between "Salafism," the movement dedicated to the example of the first generations of Muslims and to ameliorative reform, and "Wahhabism," the violent and coercive Saudi-sponsored jihadist movement behind 9/11--terms incorrectly used by journalists and academics as synonymous. (Contains 13 footnotes.)