Because those in power define who are the terrorists.

True, but you don’t have to parrot what those in power tell you to say. Use their own terms against them, or use a completely different terminology - just make sure you’re the one who defines the terms, and that you’re the one to create context (i.e. meaning). Gather enough people who share the world-view you’ve created and you have a chance to overthrow existing power structures. But even if you don’t, you’ll be “free” - at least in the mental sense. Of course, depending on how radically different your views are from the status quo, you might also seem insane to some people (in which case its probably best to smile, nod and agree to what others are saying)