A ghost is said to be the apparition of a deceased person, frequently similar in appearance to that person, and usually encountered in places she or he 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 hauntings.
According to a poll conducted in 2005 by the Gallup Organization, about 32% of Americans believe in the existence of ghosts in spite of the fact that there is no scientific evidence for their existence.
The belief in ghosts as souls of the departed is closely related to the ancient concept of animism, which attributed souls to everything in nature, including human beings, animals, plants, rocks, etc.  As the nineteenth-century anthropologist James Frazer explained in his classic work, The Golden Bough, souls were seen as the creature within that animated the body:
"If a man lives and moves, it can only be because he has a little man or animal inside, who moves him. The animal inside the animal, the man inside the man, is the soul. And as the activity of an animal or man is explained by the presence of the soul, so the repose of sleep or death is explained by its absence; sleep or trance being the temporary, death being the permanent absence of the soul... "
Although the human soul was sometimes symbolically or literally depicted in ancient cultures as a bird or other animal, it was widely held that the soul was an exact reproduction of the body in every feature, even down to clothing the person wore. This is depicted in artwork from various ancient cultures, including such works as the Egyptian Book of the Dead, which shows deceased people in the afterlife appearing much as they did before death, including the style of dress.
Another widespread belief concerning ghosts is that they were composed of a misty, airy, or subtle material. Anthropologists speculate that this may also stem from early beliefs that ghosts were the person within the person, most noticeable in ancient cultures as a person's breath, which upon exhaling in colder climates appears visibly as a white mist. This belief may have also fostered the metaphorical meaning of "breath" in certain languages, such as the Latin spiritus and the Greek pneuma, which by analogy became extended to mean the soul. In the Bible, God is depicted as animating Adam with a breath.
Although the evidence for ghosts is largely anecdotal, the belief in ghosts throughout history has remained widespread and persistent.
In many historical accounts, ghosts were thought to be deceased people looking for vengeance, or imprisoned on earth for bad things they did during life. Most cultures have ghost stories in their mythologies. Many stories from the Middle Ages and the Romantic era rely on the macabre and the fantastic, and ghosts are a major theme in literature from those eras.
Ghost stories date back to ancient times, and can be found in many different cultures. The Chinese philosopher, Mo Tzu (470-391 BC), is quoted as having said:
"The way to find out whether anything exists or not is to depend on the testimony of the ears and eyes of the multitude. If some have heard it or some have seen it then we have to say it exists. If no one has heard it and no one has seen it then we have to say it does not exist. So, then, why not go to some village or some district and inquire? If from antiquity to the present, and since the beginning of man, there are men who have seen the bodies of ghosts and spirits and heard their voices, how can we say that they do not exist? If none have heard them and none have seen them, then how can we say they do? But those who deny the existence of the spirits say: "Many in the world have heard and seen something of ghosts and spirits. Since they vary in testimony, who are to be accepted as really having heard and seen them?" Mo Tzu said: As we are to rely on what many have jointly seen and what many have jointly heard, the case of Tu Po is to be accepted." (note: King Hsuan (827-783 BC) executed his minister, Tu Po, on false charges even after being warned that Tu Po's ghost would seek revenge. Three years later, according to historical chronicles, Tu Po's ghost shot and killed Hsuan with a bow and arrow before an assembly of feudal lords.)
One of the earliest known ghost "sightings" in the west took place in Athens, Greece. Pliny the Younger (c. (50 AD) described it in a letter to Licinius Sura: Athenodoros Cananites (c. 74 BC – 7 AD), a Stoic philosopher, decided to rent a large, Athenian house, to investigate widespread rumors that it was haunted. Athenodoros staked out at the house that night, and, sure enough, a disheveled, aged spectre, bound at feet and hands with rattling chains, eventually "appeared". The spirit then beckoned for Athenodoros to follow him; Athenodoros complied, but the ghost soon vanished. The philosopher marked the spot where the old man had disappeared, and, on the next day, advised the magistrates to dig there. The man's shackled bones were reportedly uncovered three years later. After a proper burial, the hauntings ceased.
The Hebrew Torah and the Bible contain few references to ghosts, associating spiritism with forbidden occult activities cf. Deuteronomy 18:11. The most notable reference is in the First Book of Samuel (I Samuel 28:7-19 KJV), in which a disguised King Saul has the Witch of Endor summon the spirit of Samuel. In the New Testament, Jesus has to persuade the Disciples that he is not a ghost following the resurrection, Matthew 24. In a similar vein, Jesus' followers at first believe him to be a ghost when they see him walking on water.
The Child ballad Sweet William's Ghost recounts the story of a ghost returning to beg a woman to free him from his promise to marry her, as he obviously cannot being dead; her refusal would mean his damnation. This reflects a popular British belief that the dead would haunt their lovers if they took up with a new love without some formal release.
The Unquiet Grave expresses a belief even more widespread, found in various location over Europe: ghosts can stem from the excessive grief of the living, whose mourning interferes with the dead's peaceful rest.
In many folktales from around the world, the hero arranges for the burial of a dead man. Soon after, he gains a companion who aids him and, in the end, the hero's companion reveals that he is in fact the dead man. Instances of this include the Italian fairy tale Fair Brow and the Swedish The Bird 'Grip'.
 Skeptical analysis
Critics of "eyewitness ghost sightings" suggest that limitations of human perception and ordinary physical explanations can account for such sightings; for example, air pressure changes in a home causing doors to slam, or lights from a passing car reflected through a window at night. Pareidolia, an innate tendency to recognize patterns in random perceptions, can cause people to believe they have seen ghosts. Reports of ghosts "seen out of the corner of the eye" may be accounted for by the sensitivity of human peripheral vision. According to skeptical investigator Joe Nickell:
...peripheral vision is very sensitive and can easily mislead, especially late at night, when the brain is tired and more likely to misinterpret sights and sounds.
Nickell also states that a person's belief that a location is haunted may cause them to interpret mundane events as confirmations of a haunting:
Once the idea of a ghost appears in a household . . . no longer is an object merely mislaid. . . . There gets to be a dynamic in a place where the idea that it's haunted takes on a life of its own. One-of-a-kind quirks that could never be repeated all become further evidence of the haunting.
Sound is thought to be another cause of supposed sightings. Frequencies lower than 20 hertz are called infrasound and are normally inaudible, but scientists Richard Lord and Richard Wiseman have concluded that infrasound can cause humans to experience bizarre feelings in a room, such as anxiety, extreme sorrow or even the chills.
Another potential explanation of apparitions is that they are hypnagogic hallucinations.
The traditional perception of ghosts wearing clothing is considered illogical by some researchers, given the supposed spiritual nature of ghosts, suggesting that the basis of what a ghost is said to look like and consist of is quite dependent on preconceptions made by society. Skeptics also say that, to date, there is no credible scientific evidence that any location is inhabited by spirits of the dead.
Some researchers, such as Professor Michael Persinger (Laurentian University, Canada), have speculated that changes in geomagnetic fields (created, e.g., by tectonic stresses in the Earth's crust or solar activity) could stimulate the brain's temporal lobes and produce many of the experiences associated with hauntings. This theory has been tested in various ways. Some scientists have examined the relationship between the time of onset of unusual phenomena in allegedly haunted locations and any sudden increases in global geomagnetic activity. Others have investigated whether the location of alleged hauntings is associated with certain types of magnetic activity. Finally, a third strand of work has involved laboratory studies in which stimulation of the temporal lobe with transcerebral magnetic fields has elicited subjective experiences that strongly parallel phenomena associated with hauntings. All of this work is controversial; it has attracted a large amount of debate and disagreement.
Perhaps the most recognizable ghost in English literature is the shade of Hamlet's father in the play The Tragical History of Hamlet, Prince of Denmark. In Hamlet, it is the ghost that encourages the title character to investigate his "murder most foul" and seek revenge upon King Claudius, the suspected murderer of Hamlet's father.
Possibly the next most famous apparitions are the ghosts of A Christmas Carol, where the ghost of Jacob Marley, The Ghost of Christmas Past, The Ghost of Christmas Present and The Ghost of Christmas Yet to Come help Ebenezer Scrooge see the error of his ways.
Oscar Wilde's The Canterville Ghost has been adapted for film and television on several occasions. Henry James's The Turn of the Screw has also appeared in a number of adaptations, notably the film The Innocents and Benjamin Britten's opera The Turn of the Screw. Noel Coward's play Blithe Spirit, later made into a film, places a more humorous slant on the phenomenon of haunting of individuals and specific locations.
Films including or centering on ghosts are common, and span a variety of genres. Ghosts can also be found in various television programs.
The ghost hunting theme has also become prevalent in reality television series particularly Ghost Hunters and Ghost Hunters International, but also Most Haunted, and A Haunting. It is also represented in children's television by such programmes as The Ghost Hunter.
The Ghost in the Shell series is a Science-fiction futuristic cyber-punk drama written by acclaimed Manga author Masamune Shirow (who also authored the equally popular “Appleseed” Manga series). The “Ghost” in the title is a translation of the Japanese concept for soul.
In his works (Ghost in the Shell, Appleseed) Shirow questions what it is that comprises a self-aware consciousness, or an individual soul. Technology blurs the line and plays with the consequences of unrestrained advancement and asks the question can the soul be moved from one shell, the body of human birth; into a superior mechanical android or bionic body?
Through-out his works, his characters search for technological equilibrium; for personal and societal balance between technology and artificial intelligence. A key theme running through his works is the question of how can technology enhance rather than interfere with the quality of human existence and the consequences of when there is no balance.
Throughout his fiction, Shirow covers much of the same territory as American author Philip K. Dick (December 16, 1928 – March 2, 1982) who explored what it is that makes a human uniquely human; Shirow often explores the same sociological, political, metaphysical and religious meanings of existence but confines these discussions within the framework of post-cyberpunk settings.
The first installment of Ghost in the Shell (攻殻機動隊, Kōkaku Kidōtai, translated as "Mobile Armored Riot Police") was published in serial format in 1989 in the “Young Magazine”. A sequel, Ghost in the Shell 2: Man/Machine Interface was released in 2002. It has spawned two theatrical movies Ghost in the Shell and Ghost in the Shell 2: Innocence; and three TV Anime movies, Ghost in the Shell: Stand Alone Complex: The Laughing Man; and Ghost in the Shell: Stand Alone Complex: Individual Eleven and the 2008 movie Ghost in the Shell: Stand Alone Complex: Solid State Society.
It was recently announced on Monday, April 14, 2008 in Variety that Steven Spielberg will make a big-budget live-action movie of Ghost in the Shell. No release date has been set yet. (see: http://www.variety.com/VR1117984029.html)