It is simple and well-known, still some people seem to have difficulties with the concept.

It does not depend on the cause you fight for. A cause which seems fair enough to some, may look unacceptable to others, and sometimes it is extremely difficult (not to say impossible) to decide who's right and who's not. See Northern Ireland, Lebanon, Palestine, etc.

So the nature of your fight does not depend on what you fight for; it depends on how you fight for it. The thin red line between Terrorism and Resistance has a simple name : it is called innocent civilians. The minute you intentionally kill civilians in order to reach your goals, whatever they are, you become a terrorist. It's as simple as that.

This is the most commonly used definition (and in fact the only one) for terrorism in international diplomacy.

Note that this is not a permanent doom: being a terrorist at some time does not prevent you from promoting peace later on. See Yasser Arafat (Peace Nobel Prize winner).

Note also that from this viewpoint, XXth century wars amount to little more than State-sponsored terrorism - although the scale usually differs.