Definitions for "Chromatic aberration"
An optical defect in a lens resulting in different wavelengths of light focusing at different distances from the lens, which can be seen as halos around the image.
failure of lens to bring light of different wavelengths to a common focus . This can be compensated by using an achromatic lens.
Refractor telescopes cannot bring all colors to a single point of focus, and consequently there is always some amount of secondary color seen in a refractor, especially at high magnifications. Think of how a prism sperates light into a spectrum; simple lenses do this, too, and so complex multi-element lens designs are needed to minimize this. The inability to focus all colors to a common point is called chromatic abberation. Purely reflecting telescopes have zero chromatic abberation.