Intent is a word I used, but not as well as this guy did.
from a post on Daniel-in-Brookline's blog
Terrorism,  is a specific decision not to abide by any rules of combat -- and, in fact, to use any and all aspects of civilization against the victims, while simultaneously claiming the protections of civiliation for itself. 
That's why terrorists use tactics that no self-respecting professional military organization would use. That's why terrorists fight under the cover of ambulances, places of worship, and the white flag of surrender... yet scream bloody murder if any of those sanctuaries are harmed in the slightest, even by accident. That's why terrorists deliberately target civilians, while soldiers understand that their purpose is to fight other soldiers in order to protect the civilians of both sides.
He also points to Bill Whittle's essay on Sanctuary, which I read when he posted it. It is an excellent essay.