"Terrorism is an anxiety-inspiring method of repeated violent action, ... whereby...the direct targets of violence are not the main targets. The immediate human victims of violence are generally chosen randomly (targets of opportunity) or selectively (representative or symbolic targets) from a target population, and serve as message generators" (Schmid and Jongman 1988).Or consider this:
The purpose of terrorism is to exploit the media in order to achieve maximum attainable publicity as an amplifying force multiplier in order to influence the targeted audience(s) in order to reach short- and midterm political goals and/or desired long-term end states.
Terrorists do not primarily aim at the physical, but on the strategy that primarily relies on the symbolic strength of the act. The use of terror serves not primarily the purposes of fighting, injuring or destroying the opponent. Rather, its primary purpose lies in the conveying of messages to the target audience(s). Terrorists perpetrate their acts without regard for war fighting conventions. The symbolism originating from terrorist acts and media marketing thereof is intended to address the public, to use them as a vehicle and a communication channel to influence the political representatives/decision makers and other target audiences.
Considering the above, the "why" of terrorism--its intended purpose--appears to have wide or even universal agreement. But other aspects of the definition, dealing with "who" terrorists are, and "how" they attack, find less agreement. For example, some terrorism definitions (such as that used by the FBI) require that a violent act be perpetrated against innocent civilians, while other standards are more extensive and accept not just actual violence but also symbolic violence, or acts that threaten future violence.
Another point of disagreement centers on the "who" of terrorism, in that some definitions exclude individual actors, or others might exclude state actors. An example of the latter case was related by former US Ambassador Edward Peck to Democracy Now:
"1985, when I was the Deputy Director of the Reagan White House Task Force on Terrorism, they asked us—this is a Cabinet Task Force on Terrorism; I was the Deputy Director of the working group—they asked us to come up with a definition of terrorism that could be used throughout the government. We produced about six, and each and every case, they were rejected, because careful reading would indicate that our own country had been involved in some of those activities.
After the task force concluded its work, Congress got into it, and you can google into U.S. Code Title 18, Section 2331, and read the U.S. definition of terrorism. And one of them in here says—one of the terms, "international terrorism," means "activities that," I quote, "appear to be intended to affect the conduct of a government by mass destruction, assassination or kidnapping."
Yes, well, certainly, you can think of a number of countries that have been involved in such activities. Ours is one of them. Israel is another. And so, the terrorist, of course, is in the eye of the beholder."