This article is from the Photographic Lenses FAQ, by David Jacobson with numerous contributions by others.
The distance behind the lens at which monochromatic light (light of a single wavelength) comes to focus varies as a smooth function of the wavelength. If this function has a zero derivative in the visible range, and hence if there are two wavelengths at which the light comes to focus in the same plane, the lens is called achromatic. If there is a higher order correction, usually with the result that 3 or more visible wavelengths come to focus at the same distance, the lens is called apochromatic. Some authorities add more conditions. Apochromatic lenses often contain special low-dispersion glasses. APO is an abbreviation for apochromatic.
It is frequently asserted in the rec.photo.* newsgroups that marketeers use the terms apochromatic and APO rather loosely.