The modern idea of terrorism derives from the bomb-throwing anarchists of the 19thCE. Their philosophy (?!) was that society is unreformable, it must be brought crashing down and from the rubble would sprout a truly free fairy-land of reason and guilt-free sex. The basic foundation of society is security, we exchange some of our freedom for protection. Bomb-throwers calculated that modern societies were too fragile to provide that security, and with that foundation undermined they would collapse. Recent examples of this thinking can be found in the Baader-Meinhof/RAF and Red Brigades era, for all their Marxist pretensions. (Terrorism is ideological anathema to Marxism-Leninism.)
Naturally the term was used to demonise any politically-motivated violence, even when it had a more rational strategy. The IRA (IRB at the time) London bombing campaign of the late 19thCE didn't have millennialist conceits, it was expected to force a change in Britain's Irish policy but it was still labelled terrorist. The Nazis labelled the French Resistance as terrorists. The British called Begin a terrorist (as did Ben Gurion).
Where do OBL and his pack of imps fit in this analysis? I would say quite definitely that they are terrorist. They seek to bring modern society crashing down (to the level it did in Afghanistan) and see it as fragile. They have an enormous self-regard, they really think their sporadic and opportunistic attacks - which are terribly important to them - can really make a difference in the world. And they will fade away, because normal people like society, they like security, they have chosen to evolve social systems to live in. They will respond to random violence by surrendering even more of their freedom to the social system, if it's that freedom which is being exploited by the terrorists. Or even if that's just what they're told. The end result is that society becomes more robust