Rough difference.
2.5 D is where the Z axis drops to a predetermined depth and the shape is produced in X and Y whilst at this depth. X and Y can work together like doing a circle and it's called interpolation.

Typical example is doing a pocket down to 10mm deep in stages of 2mm at a time.

3D is where the Z axis is changing all the time as the X and Y are moving, imagine machining the aerofoil section on a plane wing.

Virtually all machines with a powered Z can do 2.5D and 3D, the difference is in the CAM software to generate the code.

2.5D is relatively cheap but full 3D starts to get very expensive.

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