See Halo Effect.
The term halo also refers to a defect of screen printing characterized by a faint doubling of the printed image, caused by movement of the stencil under the pressure of the squeegee, typically due to an improperly tensioned screen fabric.
A halo is also used in screen printing to refer to an enlarged halftone dot, stemming from one of many causes. (Also known as fringe.)
In halftone photography, a halo is a characteristic of a soft dot. See Soft Dot.