Chromatic Aberration
Definition
The color fringing in cheap lenses — different colors focus at different points because glass bends them by different amounts.
Technical definition
An optical defect from dispersion: refractive index varies with wavelength, so focal length differs per color (blue focuses closer than red). Corrected with achromatic doublets combining crown and flint glass.
Example
Purple-green fringes along high-contrast edges in photos — that is chromatic aberration from the camera lens failing to focus all colors together.
Common confusion
Chromatic aberration is about color (wavelength), not lens shape — that is spherical aberration. A perfectly shaped lens still shows chromatic aberration if made of a single dispersive glass.