An apparition is what most people think of when they imagine a ghost (in actuality, there are three different categorizations of ghosts: Poltergeists, Apparitions, and Haunts). Apparitions tend to appear in the same place, and typically repeat certain actions over and over. They take no notice of their surroundings and do not attempt to communicate with the living.
Many parapsychologists and other researchers believe that some kind of impression or vibration causes these apparitions, which are recorded by the place itself. They speculate that a strong emotional moment has been captured in a sort of “spiritual videotape” that is constantly replayed.
Another, less scientific, theory is that apparitions are indeed the spirits of people who died violently or unexpectedly. The apparitions may not even realize that they are dead. They are forced to repeat their last actions until released by some form of exorcism.