The primary hallmark of terrorism is targeting civilians (as opposed to military or infrastructure targets such as trains or refineries or factories). If a drone is used to attack a factory or a military target that is not terrorism. If a drone is intentionally used to attack a civilian, non-infrastructure target (eg, a wedding) then that is terrorism.
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.
