In folklore, a ghost is the soul or spirit of a dead person or non-human animal that is believed by some people to be able to appear to the living. In ghostlore, descriptions of ghosts vary widely, from an invisible presence to translucent or barely visible wispy shapes to realistic, lifelike forms. The deliberate attempt to contact the spirit of a deceased person is known as necromancy, or in spiritism as a séance. Other terms associated with it are apparition, haunt, haint, phantom, poltergeist, shade, specter, spirit, spook, wraith, demon, and ghoul.

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 other than humans have also been recounted. They are believed to haunt particular locations, objects, or people they were associated with in life. According to a 2009 study by the Pew Research Center, 18% of Americans say they have seen a ghost.

The overwhelming consensus of science is that there is no proof that ghosts 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 the spirits of the dead. Historically, certain toxic and psychoactive plants (such as datura and hyoscyamus niger), whose use has long been associated with necromancy and the underworld, have been shown to contain anticholinergic compounds that are pharmacologically linked to dementia (specifically DLB) as well as histological patterns of neurodegeneration. Recent research has indicated that ghost sightings may be related to degenerative brain diseases such as Alzheimer's disease. Common prescription medication and over-the-counter drugs (such as sleep aids) may also, in rare instances, cause ghost-like hallucinations, particularly zolpidem and diphenhydramine. Older reports linked carbon monoxide poisoning to ghost-like hallucinations.

In folklore studies, ghosts fall within the motif index designation E200–E599 ("Ghosts and other revenants").

The English word ghost comes from Old English gāst ("breath, spirit, soul, ghost"), which can be traced back to Proto-Germanic *gaistaz ("spirit, ghost"). It is cognate (linguistic sibling from a common origin) with Old Frisian gāst ("spirit, ghost, demon"), Old Saxon gēst ("soul, vitality, spirit, demon"), Old Dutch gēst ("spirit"), and Old High German geist ("spirit"). Although recorded descendants do not appear in North and East Germanic sources (where Gothic uses ahma and Old Norse uses andi m. or önd f.), linguists reconstruct *gaistaz as stemming from pre-Germanic *ghois-t-oz ("fury, anger"). This reconstruction is supported by its connection to Sanskrit hīḍ- ("to be angry") and héḍa ("anger"), and to Avestan zōižda- ("terrible"; in zōiždišta "most terrible").

The common Proto-Indo-European form is posited as *ǵʰoys-d-os, a dental suffix derivative of the root ǵʰéys-. This root also appears Proto-Germanic *gaistjan ("to terrify"; compare Old English gǽstan and Gothic usgaisjan), in Old Norse *geiski ("fear"; implied in geiskafullr, "full of fear"), and in Avestan zōiš- (in zōišnu, "shivering, trembling").

Besides denoting a "person's spirit or soul" (as "the life force" or "breath of life" that gives life to the body, in contrast to its purely material being), the Old English word is also used as a synonym of Latin spīritus in the meaning of "the breath of God or a god" from the earliest attestations (9th century). It could also denote any good or evil spirit, such as angels and demons (the Anglo-Saxon gospel refers to the demonic possession of Matthew 12:43 as se unclæna gast). Also from the Old English period, the word could denote the spirit of God, the "Holy Ghost" (halgan gaste), after post-classical Latin spiritus sanctus .

The now-prevailing sense of "the soul of a deceased person, spoken of as appearing in a visible form" only emerges in Middle English (14th century). The modern noun does, however, retain a wider field of application, extending on one hand to "soul", "spirit", "vital principle", "mind", or "psyche", the seat of feeling, thought, and moral judgement; on the other hand used figuratively of any shadowy outline, or fuzzy or unsubstantial image; in optics, photography, and cinematography especially, a flare, secondary image, or spurious signal.