In mythology and folklore, a vengeful ghost or vengeful spirit is said to be the spirit of a dead person who returns from the afterlife to seek revenge for a cruel, unnatural or unjust death. In certain cultures where funeral and burial or cremation ceremonies are important, such vengeful spirits may also be considered as unhappy ghosts of individuals who have not been given a proper funeral.

In folklore, a ghost is the soul or spirit of a dead person or non-human animal that is believed 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.