APO

Apochromatic - the lens focuses 3 frequencies of light
(probably Red, Green and Blue) to the focal plane.
This helps minimize color fringing.

A lesser quality lens might be Achromatic, focusing 2 frequencies to the
focal plane. You would get color fringing of the third primary color.

Note that just because a lens is Apochromatic, that doesn't say anything
about what 3 frequencies it focuses at the focal plane or anything about
the performance of any other frequencies at all. (it would be possible if
stupid to create an Apochromatic lens that focused 3 infrared frequencies
at the focal plane, but focused red, green and blue at incompatible
distances - such a lens would be totally useless for visible light
photography)