I think "terror" can't help but be about perception, regardless of legal definitions. If only a single attack per year kills a civilian through accident, yet it is dramatic and publicized every time, it can still lead to a reaction of public terror, even if it is a less likely cause of death than slipping in the bathroom. Suicide bombers and UAVs are equally capable of creating this mass psychological effect; intent (both real and perceived) is a secondary factor.

This is a very modern definition. Most historical terrorism has been committed against property, rather than civilians.