A ghost light is an electric light that is left lit on the stage of a theater when the theater is unoccupied and would otherwise be completely dark. It typically consists of an exposed incandescent bulb mounted in a wire cage on a portable light stand.
The practical use for a ghost light is for safety. Sometimes also known as equity lights or equity lamps, the lights often keep workers from falling into the pit or tripping on scenery left onstage in-between shows.