A ghost light is an electric light that is left energised on the stage of a theatre when the theatre is unoccupied and would otherwise be completely dark
The practical use of a ghost light is for safety.
Some claim that the tradition began in the days of gas-lit theatres when dim gas lights were left burning to relieve pressure on the gas valves.
The superstitious have various justifications for the ghost light in relation to the supernatural. A popular theatrical superstition holds that every theatre has a ghost, and some theatres have traditions to appease ghosts that reach far back into their history