The Encyclopedia Britannica defines terrorism as "the systematic use of violence to create a general climate of fear in a population and thereby to bring about a particular political objective." What distinguishes terrorism from conventional warfare is the degree to which its efforts are focused on creating public panic. While conventional military forces also make frequent use of psychological warfare, including acts of terror and various forms of propaganda, they seek to achieve victory principally through strength of arms.

Terrorists, by contrast, often believe that they cannot win the battle by sheer force of arms because they are fighting a technologically and militarily superior foe. Thus they focus their efforts on covert actions designed to strike the constant fear of unpredictable violence into the hearts of all - particularly civilians. This has led some social scientists to refer to terrorism as the "weapon of the weakest." Attacking civilians has been the methodology. These terrorists pursue their political goals by striving to make life unbearable for the population at large - and thereby winning political concessions.

In some cases, of course, terrorism is a tool of the mighty - such as when it has been the official policy of totalitarian states like Hitler's Nazi Germany and Stalin's Soviet Union. Those states used unjustified arrest, imprisonment, torture, and execution to create a climate of intense fear, and to encourage subservience to the declared political, social, and economic goals of the state.

In order to attract and maintain the publicity necessary to generate widespread fear, terrorists commonly engage in increasingly dramatic, violent, and high-profile attacks. Toward this end, they often target buildings or other locations that are important economic or political symbols, such as embassies or military installations. Most notable were the 9/11 attacks against the World Trade Center and the Pentagon.

Technological advances in the weaponry of recent decades, coupled with the efforts of terrorist organizations to acquire such arsenals (particularly chemical, biological, and nuclear), give present-day terrorists an unprecedented potential for lethality. Moreover, it is clear that Islamic terrorist groups in particular are willing and eager to use weapons of mass destruction. This was expressed in a 2002 al Qaeda manifesto which candidly declared, "We have not reached [military] parity with them [the Americans]. We have the right to kill 4 million Americans -- 2 million of them children -- and to exile twice as many and wound and cripple hundreds of thousands. Furthermore, it is our right to fight them with chemical and biological weapons . . ." 

The term "terrorism" was first coined in the 1790s in reference to the terror used during the French Revolution by the revolutionaries against their opponents. The Jacobin party of Maximilien Robespierre carried out a Reign of Terror involving mass executions. Although terrorism in this usage connotes state violence against its domestic enemies, nowadays the term is used most frequently to describe violence designed to influence government policy or topple an existing regime.

While there is no universal agreement about precise definitions of terrorism, one popular typology identifies three broad classes of terrorism:

(a) Revolutionary terrorism, which seeks the total abolition of a political system and its replacement with something new;

(b) Sub revolutionary terrorism, which is aimed not at overthrowing an existing regime but rather at modifying the existing sociopolitical structure; and