In folklore, a ghost (sometimes known as an apparition, haunt, phantom, poltergeist, shade, specter or spectre, spirit, spook, and wraith) is the soul or spirit of a dead person or animal that can appear to the living. Descriptions of ghosts vary widely from an invisible presence to translucent or barely visible wispy shapes, to realistic, lifelike visions. The deliberate attempt to contact the spirit of a deceased person is known as necromancy, or in spiritism as a séance.

The belief in the existence of an afterlife, as well as manifestations of the spirits of the dead is widespread, dating back to animism or ancestor worship in pre-literate cultures. Certain religious practices—funeral rites, exorcisms, and some practices of spiritualism and ritual magic—are specifically designed to rest the spirits of the dead. Ghosts are generally described as solitary, human-like essences, though stories of ghostly armies and the ghosts of animals rather than humans have also been recounted. They are believed to haunt particular locations, objects, or people they were associated with in life.

The overwhelming consensus of mainstream science is that ghosts do not exist. Their existence is impossible to falsify, and ghost hunting has been classified as pseudoscience. Despite centuries of investigation, there is no scientific evidence that any location is inhabited by spirits of the dead.

While deceased ancestors are universally regarded as venerable, and often believed to have a continued presence in some form of afterlife, the spirit of a deceased person that persists in the material world (a ghost) is regarded as an unnatural or undesirable state of affairs and the idea of ghosts or revenants is associated with a reaction of fear. This is universally the case in pre-modern folk cultures, but fear of ghosts also remains an integral aspect of the modern ghost story, Gothic horror, and other horror fiction dealing with the supernatural.

A place where ghosts are reported is described as haunted, and often seen as being inhabited by spirits of deceased who may have been former residents or were familiar with the property. Supernatural activity inside homes is said to be mainly associated with violent or tragic events in the building's past such as murder, accidental death, or suicide—sometimes in the recent or ancient past. But not all hauntings are at a place of a violent death, or even on violent grounds. Many cultures and religions believe the essence of a being, such as the 'soul', continues to exist. Some religious views argue that the 'spirits' of those who have died have not 'passed over' and are trapped inside the property where their memories and energy are strong.