As with nationalism, terrorism is a modernist construct. How can this be? Weren't there always terrorist acts? I would suggest that there weren't. People were filled with terror by many actions of pre-modern societies, but it can't be called terrorism in the sense that we mean it today. What kind of terror does terrorism inflict? A superficial analysis might be that it is the loss of life, but in fact it is the loss of subjectivity, or a particular kind of life.
a) Totalization of reality: It explains why we are in the state that we are in. Terrorism justifies an "us-vs-them" mentality. It enables the definition of nation. Terrorism gives a modernist focus to nationhood, in that it clarifies the moral rightness of the nation. Of course, terrorists themselves may not be nationalists (Osama bin Laden isn't, for instance), but the threat is regarded as a nation-surrogate.
b) The Rational Self: Terrorism would seem to be the most irrational of actions, but in fact it has a clear end or goal (even if that end is only the toppling or frustration of the enemy). To treat terrorism as terrorism, rather than as criminal action or the threat of war by another nation, is to recognize that goal and meet it with goals of one's own.
c) The Principle of Legitimation: It legitimates certain practices on the basis of the interests of the state (e.g., tolerance of restrictive measures such as the Patriot Act);
d) Abstraction: The actions of terrorists are not abstract, of course, but the goals and beliefs that motivate them may well be. And, those goals/beliefs are met by other goals and beliefs among the victims of the attacks.
e) Emancipation: Terrorism is rarely seen as an end in itself, but rather as a means to an end. That end is the alleviation of a problem. Of course, that problem might be misidentified, but that is the story that the terrorists tell themselves. For those attacked, then, there is also emancipation, which is the elimination of the terrorists. It is worth noting, though, that since terrorism is often not identified with a specific nation, that emancipation can be an ever-moving target, and the legitimation of measures leading to that end can have an indefinite endpoint.