This is an exceedingly difficult term to define. I would define it as violent action that is taken against targets that are not military targets within a war zone and that is aimed at promoting a particular social or political goal or cause that the person defining terrorism does not approve of. Let us look at each aspect of this definition.
- Violent action. Terrorism must involve violence, and will typically involve lethal violence. It must do so because it is meant to terrorize people, not to reason with them. Therefore, any sort of speech, even if it takes the form of threats that are not backed by action, cannot really be seen as terrorism.
- Nature of the targets. An attack on a military base that is in a war zone cannot really be seen as terrorism. However, the 9/11 attack on the Pentagon was a terrorist act because those military people would not have expected that their lives could be in danger in the way that soldiers in Afghanistan, for example, would.
- Social or political goals. The attacks cannot be violence for the sake of violence. A serial killer is not a terrorist nor is a school shooter. There must be an attempt to promote a certain ideology. That is why the Boston Marathon bombing can be seen as an act of terrorism where the Connecticut school shooting cannot be.
- Nature of the goal. We typically never call something terrorism if we agree with the goals being sought. If the goals being sought are ones we agree with, we see the actions as legitimate. That is, for example, why we do not call John Brown or Nat Turner “terrorist leaders.”
There are other aspects to terrorism that could be included, but these are the aspects that I think are most important.