So the old chestnut what is a terrorist is back again - and will be for the rest of time (& rightly so in my opinion). To my mind, terrorists are people fighting for a cause they believe in; but are not represented by an official government, although they can be supported by one. Their aim is to spread their doctrine through the use of coercion, fear and generally being thoroughly nasty chaps. They won't play ball with the establishment and turn up to a decent pitched battle - because they know that they will lose. A freedom fighter, is the moniker giiven to a terrorist, by governments and people sympathetic to the doctrine that the "terrorist" is trying to spread.
Since Centurion was a rank, not a tank, and before the powers that be have sought to extend their influence through the use of "terrorists" to extend their influence, or keep competitors destabilised. There is no government that has clean hands, because it is a cheap way to wage war, sell arms and generally screw your competitiors over, covertly.
To really get to grips with the question of terrorism, you need to examine each case, who is involved, what are they seeking and who benefits - that's where you need to focus your energy.
As in most things in life, broad brush statements can simplify explanations, but they muddy the discussion - this is my humble opinion anyway.