A ghost light is a light which is left burning in a theater after everyone goes home for the night. There are a number of explanations for the light, ranging from the supernatural to the mundane, and such lights are in common use around the world. Many theater people also use the term to refer to any sort of bare bulb left burning at light in an establishment after it closes, whether or not the establishment is a theater.
The obvious practical reason for using a ghost light is that it reduces the risk of injury for someone who is in the theater after it closes. Sometimes theater personnel work late, or come in early, and the light can help them navigate until they turn additional lights on. Stages have a number of areas where people could be potentially injured by falling or tripping, and ghost lights reduce this risk. A light can also serve as a liability protection in the event that the theater is broken into, because if a burglar becomes injured while the ghost light is on, the theater may be able to claim that it is not responsible.
The superstitious tradition behind the light is ancient, dating back to at least the time of Shakespeare, and probably even older periods in theatrical history. According to legend, a candle would be left burning on the stage to dispel the ghosts of previous performances, especially if those performances had gone badly. The candle may also well have deterred people trying to break into the theater, of course, by suggesting that someone was present.
Modern theaters usually use a ghost light for liability reasons, but they may use a more mystical explanation for the light's presence. Some say that lights are left on to allow the ghosts in the theater to perform at night, for example, while others fall back on the tradition that the light is supposed to keep ghosts away. These conflicting superstitions about the ghost or spook light illustrate the complex and very superstitious culture which surrounds theatrical performance.
A traditional ghost light consists of a bare bulb, sometimes surrounded with a cage to reduce the risk of fire. The bulb is mounted on a movable pole or stand which is carried onto the stage at night and plugged into an outlet which is left on. Some theaters prefer to hang their lights from the ceiling, simply turning them on at night as staff leave rather than going through the ritual of carrying out the ghost light and plugging it in.