Glenn Greenwald, one of the brightest and best thinker/writers of our time, articulates what (I imagine) so many of us feel about the definition of terrorist but can't quite put into words.
"In other words, if the U.S. invades and occupies your country, and you respond by fighting back against the invading army — the ultimate definition of a “military, not civilian target” — then you are a . . . Terrorist."