Chromatic Aberration, also referred to as color fringing, is the addition of a colored line or halo (often purple, green or blue) around certain subjects in an image.
It’s a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point.
Notice the purple fringing on the left side.
And the turquoise fringing around the photographer.
For a more detailed overview, check out Wikipedia.