In a nutshell...because you can get better answers in the 3D forum:

The simplest definition is flight below stall speed (that's NOT comprehensive). This includes things like hovering and harriers, where you're not using your wings for lift (or much lift). Most 3D manuvers rely on engine power to make them happen and your propeller becomes a major lifting agent...instead of just being used for propulsion.

I have my own definition of normal flight...I call it 2 1/2 D because you move freely in two dimensions (like you would in a boat), but fight gravity in the third. You "climb" and "dive" more so than just going "up" and "down".

Hope that helps, but like I said, there are more complete answers in the 3D forum if you do a search.

Chuck