A type of refractor objective made from lenses -- usually three -- of different materials, selected to bring three colors of light to the same focal point, thereby bringing the colors that are in between to very nearly the same focal point, thereby permitting a decent stab at being well-focused for all colors of visible light at the same time. Thus, an apochromat attempts to reduce longitudinal chromatic aberration.
An apochromat will also have spherical aberration corrected at two wavelengths of light -- usually blue and yellow -- thus attempting to reduce chromatic variation of spherical aberration. It will probably have coma corrected at one wavelength of light -- usually green.
At any rate, what preceeds is the technical definition. There is an increasing colloquial tendency to label any telescope which is free, or almost free, from chromatic aberration an apochromat. The problem with that is that advertisers have picked up on "apochromat" as a high-tech term useful in suckering buyers: Thus as the years go by, what gets labeled "apochromat", gets worse, and worse, and worse...
The problem is compounded because there are telescopes which are apochromats in the technical sense, as discussed in the preceding paragraphs, which in fact show a lot of chromatic aberration when used visually. These early apochromats had one of their "three colors" in the near ultraviolet, so that they could be focused visually to take sharp photographs with early blue/ultraviolet-sensitive photographic emulsions.

This definition is in context to Telescope. See more contextual defintions for Apochromat.