Definitions for "Apochromatic"
Free from chromatic and spherical aberration; -- said esp. of a lens in which rays of three or more colors are brought to the same focus, the degree of achromatism thus obtained being more complete than where two rays only are thus focused, as in the ordinary achromatic objective.
corrected for both chromatic and spherical aberration; "an apochromatic lens"
A lens combination in which chromatic aberration is corrected by bringing three colours to the same focus. Some manufacturers use the term to describe achromatic doublets whose false colour is approximately equivalent to that of an apochromatic triplet lens.