Close. I think the difference between 2.5 and 3 axis is that they both have 3 axis, but only 2 can move at once on a 2.5 and 3 can move at once on 3 axis.
For example, milling in either XY, XZ or YZ simultanious would be 2.5 axis, and milling in XYZ simultanious would be 3 axis.
A 3 axis machine would be a HAAS and a 2.5 might be a Bridgeport with EZTrak or something.
A 3 axis CAD/CAM package could do 3d surfacing (XYZ simultanious) like Mastercam Level 3, and 2.5 would be up to 2 axis simultanious like Mastercam Level 1, (no surfacing)