If you fight in an active civil war you are not a terrorist (1948 Arab-Israeli war)

If you strike military targets of an occupation force in a time of guerilla warfare you are not a terrorist. (Many palestinian fighters when there is an active conflict with Israel)

If you strike military targets of an occupation force in a time of relative peace, and your reignition of violence has no goal of achieving anything for your people, you are probably not a terrorist, but probably doing something wrong and stupid and horrible that hurts your own civilians, driven by nationalism or ideology or whatever. (Palestinian fighters on October 7 that struck military bases for example).

If you strike civilian targets or tage hostages, you are a terrorist. And worse if you do it at a time of relative peace to ignite violence against your own people. Hamas, Hezbollah and the Houthi's have engaged in plenty of the latter since a long time.

By the way, if you level a building with 8 militants and 20 civilians that is brutal urban warcare but not terrorism. If you go to a festival and kill predominantly hundreds of civilians, that's terrorism.