The British considered George Washington a terrorist.
(I have no idea if this fits into your discussion at all.)
I took a "History of Terrorism" course in college. The definition was presented to us by our professor throughout the semester as basically a "working definition," because it is difficult to define, it is always changing, and it is always different based on your perspective.
As has been mentioned, one man's freedom fighter is another man's terrorist.
Basically, the most concrete definition of terrorism that we got was "the use of fear by an individual or group, that sometimes includes the use of violence, to manipulate non-military individuals of the world based on a belief system."
The non-military part is important, because attacking military forces would simply be an act of war, not terrorism... but even then, different situations in this regard can get murky, and there is plenty of room for debate.
I don't think an average, everyday bully at school is a terrorist. He may be using fear and violence to manipulate his school's population, but he's doing it because he's an asshole, not for some higher belief.
But that is just MY personal view.
Terrorism is tricky, and belittling each other about what is right/wrong is silly, because even a textbook on "History of Terrorism" doesn't have a concrete definition for the word/idea.