Apochromat, or APO, is the simplified term used to refer to apochromatic lens, a photographic or other lens that has the capability to correct spherical and chromatic aberration better than the usual achromat lenses.
Chromatic aberration used to be one of the serious problems of astronomers since it messes up with telescopes that are installed with lenses instead of mirrors. It is a phenomenon wherein different colours focus at equally different distances when looked through a lens. In comparison with achromatic lenses that can bring two wavelengths to center on the same plane, apochromatic lenses are better in the sense that they are capable of handling not only two but three wavelengths: red, green and blue.
Most of the manufacturers today of apochromatic lenses meant for astrophotography are of 60-150 mm aperture range. Focal ratios range now from f/5 to f/7. In addition, not only this kind of lenses useful in astronomy; it is also widely used in photography to capture images in sharp, vivid pictures.
Apochromats are characterized by optical glasses that come with dispersive properties in order to obtain crossings of three colors. Examples of these glasses are fluoro-crown glasses and unusual flint glasses. Optically transparent liquids can be used as well, provided that they possess great dispersive properties.