Term in television broadcasting for a small, semi-transparent logo or image superimposed on the screen. This is the alpha channel version of a bug, typically located in the lower-third.
As long as the ghost contains the call letters and location of license for the station, it fulfills FCC requirements for a legal id, and this is its primary purpose. It also helps, in this day of 100+ channel cable television service, to identify what the station is to a furiously channel-flipping viewer.
A ghost can be fired automatically or manually from a keyer. There is usually a separate manual keyer that a master control operator can fire by hand in the event of failure in the automation.