What is "Terrorism"?

It's always in the news and being used as a justification for laws and policies, so what exactly is terrorism? Anthony Clark Arend has an interesting post discussing his proposed definition and how it applies to the Padilla case. He has put forth the following meaning of "terrorism":

the threat or use of force, undertaken for political purposes, by a non-state actor that intentionally targets civilians and other noncombatants in violation of existing law relating to the conduct of hostilities.

I think this is a very good definition assuming that religious motivations are included under "political purposes". A universal definition would benefit the international community as they seek to work together in preventing and punishing terrorist activities. There are currently over 100 definitions for terrorism, so I do not expect there will ever be a standardized meaning. As Arend points out in his post however, the term carries a lot of weight and should not be thrown around to prejudice suspects.