A ghost has been defined as the disembodied spirit or soul of a deceased person, although in popular usage the term refers only to the apparition of such a person. Often described as immaterial and partly transparent, ghosts are reported to haunt particular locations or people that they were associated with in life or at time of death.
Phantom armies, ghost animals, ghost trains and phantom ships have also been reported.
Ghosts or similar paranormal entities appear in film, theatre, literature, myths, legends, and some religions.
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. 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, a feeling of being watched, or even the chills. Carbon monoxide poisoning, which can cause changes in perception of the visual and auditory systems, was recognized as a possible explanation for haunted houses as early as 1921.
According to the Committee for Skeptical Inquiry, to date, there is no credible scientific evidence that any location is inhabited by spirits of the dead.
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 are reflected through a window at night. Pareidolia, an innate tendency to recognize patterns in random perceptions, is what some skeptics believe causes people to believe that 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.