Why 3D planes are known as 3D?

Is that a "simple" question ?   

I guess this mode of flying gives us one more "dimension" or "degree" of movement.  When you are hovering (i.e. "prop-hanging"), the plane is still, but still flying.

I never came across a good definition for 3D flying, most people end up saying "high-alpha" moves (at stall or very low speeds) constitute 3D flight.