CNC stands for computer numerical controller. Numerical control is the process is the automation of machine tools that are operated by precisely programmed commands encoded on a storage medium, as opposed to controlled manually by hand wheels or levers, or mechanically automated by cams alone.

Putting the two together brings forth the CNC mill. Rather than using cranks and handles to manually mill a workpiece, given the code, a CNC mill with automatically mill the part more precisely and quickly than a human could. Because of the computer control, parts can be more complex.  The more complex machines have two extra axis to allow for the rotation of the workpiece and tilt of the tool itself to allow for asymmetric and eccentric super complex designs.