The UN definition especially, but also the US State Dept. one, would have had many resistance organizations during WW2 or guerrilla fronts in Africa who were opposing colonial powers (Britain, France) or later rogue dictators (such as Idi Amin or Mobutu) - by these definitions, those organizations would have been classed as terrorists. Even if we all see their wider aims as legitimate.
The very nature of a resistance movement that has to work clandestinely and in a spread-out structure, often without a hard, rigorous leadership on all points, and without means of formal accountability up and down the lines (you stick with the people you trust and you generally follow orders without questions, or you're out, possibly dead) means there's an inherent risk of people settling private scores and dressing it up as "execution of snitches and enemy collaborators". Or raping women that are "of the enemy" or whose partners/husbands have been running errands for the enemy. After the victory that's all swept under the rug. There was a lot of all that in '44-45 in occupied France, especially after D-Day, in Spain during the civil war, on the counts of both sides, and in Central America in the 1980s, places like Honduras, El Salvador and Nicaragua (especially by the Contras and their henchmen). A lot of crappy and indefensible acts - but surely the work of the French Resistance and the Spanish Republican units was, as a whole, morally legitimate and very much needed?
It's no coincidence either that both Reagan and Thatcher classed the ANC, and Nelson Mandela, as terrorists. Yes, there were some unsavoury acts going on, in open attacks or more shady personal dealings - like the infamous "necklace killings" - but what the ANC engaged in was 95% an open and justified struggle against an evil system. And in tandem with that, the building of a civil society and civic cohesion in regions which the masters of the apartheid system wanted to keep down in a sea of filth.