Ghost
Ghosts are apparitions, often of a human size and shape, but typically described as "silvery", "shadowy", "semitransparent", "foglike", or similar. Sometimes they are not described visually, but in terms of other phenomenona, such as the movements of an object, spontaneous throwing of a lightswitch, noises etc., that supposedly have no natural explanation. According to popular accounts or myth, ghosts are disembodied spirits or souls. Beliefs about ghosts have varied over time and place, with disagreement both as to what ghosts are and whether such things exist in reality.
In the West, those who believe in ghosts sometimes hold them to be souls that could not find rest after death, and so linger on Earth. The inability to find rest is often explained by unfinished business, such as a victim seeking justice or revenge after death. Criminals sometimes supposedly linger to avoid Purgatory or Hell. It is sometimes held that ghosts reside in Limbo, a place, according to non-orthodox Catholic doctrine, between Heaven and Hell where the souls of unbaptized infants go.
In Asian cultures (such as China), many people believe in reincarnation. Ghosts are those souls that refused to be 'recycled' because they have unfinished business similar to those in western belief. Exorcists can either help a ghost to be reincarnated or blow it out of existence. In Chinese belief, apart from being reincarnated, a ghost can also become immortal and become demigod, or it can go to hell and suffer till eternality, or it can die again and become "ghost of ghost".
Skeptics seek to explain ghost sightings by applying the principle of Occam's razor, which involves selecting the simplest from all possible explanations for a phenomenon. This usually means that first, the sincerity and motive of the person reporting will be called into question. For example, lingering of ghosts is typically associated with seeking justice or revenge. Ascribing such motives and powers to dead people could be interpreted as a scare tactic directed at those who might consider murdering someone. Second, the possibility of a con will be considered, with the reporting person assumed to be the victim. It seems possible that, sometimes, the telling of ghost stories might have been a way for secluded communities to scare off intruders. It is also conceivable that, when unsuccessful, this tactic could have been backed up by more or less elaborate setups with members of that community playing ghosts. Third, explanations grounded in knowledge about human physiology will be proffered. For instance, the appearance of ghosts is often associated with a chilling sensation and pale, semitransparent figures instilling "negative energy". But a natural animal response to fear is hair-raising which can be mistaken for chill. The visual aspects could also be accounted for by human physiology: the peripheral vision is very sensitive for detecting motion, but does not contain color or provide focused shapes; therefore, a moving curtain or other movement outside the focused view can create a strong illusion of an eerie figure. Psychological factors are also often cited as natural explanations for ghost sightings: susceptible people might be prone to exaggerated interpretations of perceptions when visiting a site of unpleasant historical events.
See also: Ghost (movie), Parapsychology, Exorcism, Undead, Possession, Spectre, Ghost dance, Holy Ghost.
There is no evidence that ghosts actually exist, but they have an active life in the arts.
Ghosts in Fiction
In many stories, ghosts are often depicted as haunting the living until a certain desire is met or some grievance was settled by the haunted.
There are ghost superheroes who fight for justice such as DC Comics' The Spectre and Deadman.
In the Ghostbusters film and television franchise, the protagonists use special technology of their own design to hunt and capture/exile the ghosts they encounter.
In The Matrix, ghosts are explained as obsolete or malfunctioning programs which choose to hide in the matrix to escape deletion. A program's other option is to return to "the Source," which is like the Heaven of the matrix.
In Ghost in the Shell, ghost is a word used to describe a person's inner being, similar to the concept of a soul.
In Shakespeare's play Hamlet, a ghost taking the form of Hamlet's recently deceased father appears to Prince Hamlet one night. The ghost says that he was in fact murdered by his brother Claudius, who now (by virtue of having married Hamlet's mother Gertrude) occupies the throne. The ghost exhorts Hamlet to take revenge on Claudius. When Hamlet sees the ghost, he is not sure if it is in fact his father's spirit, or a demon whose aim is to deceive him.
One way to produce ghosts on stage is Pepper's ghost technique.
Other Meanings
In quantum field theory, ghosts are a auxiliary fields needed in non-abelian gauge theories in order to deal with the gauge freedom. Although they are real fields, ghosts follow fermionic statistics. They are only artifacts of the theory, and do not correspond to real particles.
A ghost is any image on a tv screen that doesn't belong to the programme in question.
Ghost is the name of a character in The Matrix Reloaded and The Matrix Revolutions.