A ghost is usually defined as the apparition of a deceased person, frequently similar in appearance to that person, and encountered in places he or she frequented, or in association with the person's former belongings. The word "ghost" may also refer to the spirit or soul of a deceased person, or to any spirit or demon.

Ghosts are often associated with haunting, which is, according to the Parapsychological Association, "the more or less regular occurrence of paranormal phenomena associated with a particular locality (especially a building) and usually attributed to the activities of a discarnate entity; the phenomena may include apparitions, poltergeist disturbances, cold drafts, sounds of steps and voices, and various odors."

Ghosts are controversial phenomena. According to a poll conducted in 2005 by the Gallup Organization, about 32% of Americans believe in the existence of ghosts. The term ghost has been replaced by apparition in parapsychology, because the word ghost is deemed insufficiently precise.

Ghosts are prominent in the popular cultures of various nations. Since the earliest generations of video games, ghosts have played roles in many. For instance, ghosts are the enemy in the classic game Pacman and still do in modern Pacman games. Ghosts known as Boos feature prominently in the world of Super Mario Bros. as enemies. In the related game Luigi's Mansion, the player becomes a ghost hunter.

Films including or centering on ghosts are common, and span a variety of genres. American films focused primarily on ghosts include Ghost, Ghostbusters, and Casper the Friendly Ghost.

Ghosts can also be found in various television programs that focus on the paranormal, such as the children's animated series Danny Phantom, Ghost Trackers,Truth or Scare, and Mystery Hunters. Ghostfreak from Ben 10 is another example of a ghost in an animated series. In the Harry Potter series of books and movies, Hogwarts is occupied by several ghosts. The characters of the television show Supernatural fight ghosts and other cryptids.

The ghost hunting theme has also become prevalent in reality television series such as Ghost Hunters, Most Haunted, A Haunting, and many others.

Source : http://en.wikipedia.org