APO is short for apochromatic. An apochromatic lens brings three colors of light (red, green and blue) into focus in the same plane. A simpler achromatic lens corrects only two of them (red and blue). APO lenses use special low-dispersion glass and show much less color fringing (chromatic aberration). That is why the label is common on telephoto and high-end lenses.
Do you have a better definition? Share it in the comments.
