In olden times, with resistance dimmers (see Dimmer, Resistance), if the dimmer was not loaded to within 75% of its capacity, the load would not dim completely to zero. Thus extra lights or heater coils were added to the circuit, often backstage in a corner, to make up the additional capacity.
Even today with modern phase-control dimmers, a ghost load of 100W may be needed to allow dimming of a very small load, such as a string of mini Christmas lights or a 15W practical lamp or dimmable LED replacement lamps.
AKA dummy load. Do not confuse with ghost light.