A condition where current circulates in the ground wiring system, due either to the grounds at the end of a length of cable being at different AC potential, or where a video or audio system has multiple paths to ground. Manifests itself usually by varying levels of hum (in audio systems) or as rolling bars in the picture of a video system. One way to deal with it is to use ground lift switches, but generally it is better to find the offending piece of equipment and then checking it to find out why it is causing the ground hum.