I am pretty sure the term 3D is used wrong BUT that is what has been picked so it would help to use it b/c its not going to change. Over simplified it means that its a movement thats not in the airplanes flight path (usually flight beyond the stall). Picture up and down as a dimension and left and right as a dimension. Any other movement or lack of movement or progression could be considered a third dimension.
What it boils down to is that Heli pilots came up with it. Anybody who flies a mixing blender that will chop off heads and hands is bound to come up with something outrageous . Laying all jokes aside (I'm not a heli fan since one ate through my trailer) Helis do perform 3D flight b/c of their ability to fly back wards and whatever else they do. The airplane guys just tagged our outrageous style of flying 3D after what the Heli boys do.