In traditional belief and fiction, a ghost is the soul or spirit of a deceased person or animal that can appear, in visible form or other manifestation, to the living. Descriptions of the apparition of ghosts vary widely from an invisible presence to translucent or barely visible wispy shapes, to realistic, life-like 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 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 appease the spirits of the dead. Ghosts are generally described as solitary essences that haunt particular locations, objects, or people they were associated with in life, though stories of phantom armies, ghost trains, phantom ships, and even ghost animals have also been recounted.
That's not bad. But it does underline how few of the 'ghost stories' of M.R. James deal with ghosts in the traditional sense. Indeed, a far higher ghostly strike rate was turned in by James' successors - notably Wakefield, Benson, and Burrage, who I think of as the 'inter-war boys'.