Wait a sec, I'm confused (or just underinformed). Hasn't this always been the definition of terror? Violent acts against those unable and unequipped to officially (Uncle John's shotgun on the top shelf doesn't really count) defend themselves?
Close to home examples would be that any Palestinian violence against civilians we consider terrorism. However, Palestinian violence against military targets is... well, not terrorism, that's for sure. A skirmish? Freedom fighting? Whatever.
We've never had a problem with that kind of distinction in Hebrew simply because the word most commonly translated as "terrorist" by those who don't know better is "mechabel", from the same root (and thus the same basic meaning) as the word "chabala". "Chabala" is a term one would use to describe, say, a head injury (concussion, etc). "He has chabala to his head." Simply means "damage".
Hmm, now that I think of it, I don't know if we have a proper word for terrorism - just like we don't have a proper word for assasination.