Dimensions, 2D, 2.5D, 3D

This is a basic question, but could someone explain 2D, 2.5D, 3D?

I assume 2D would work like a plotter. Moves in X and Y, with the tool
going up and down (or on and off) to fixed points.
2.5D and 3D I don't understand. Is 2.5D "profiling"? I assume that means
X, Y and Z axes that can move to any position, but isn't that 3D also?
Or does 3D incorporate 4 or more axes?

ADVthanksANCE,
JJ

Be Kind, Be Careful, Be Yourself