General terms:
* Spectre: A ghostly apparition, often seen as a frightening figure.
* Phantom: A ghost or apparition that is not easily explained.
* Spirit: A supernatural being, often associated with the soul of a deceased person.
* Apparition: A ghostly figure that appears suddenly and unexpectedly.
* Shade: A vague or shadowy ghost.
* Wraith: A ghostly figure that is often seen as a harbinger of death.
* Poltergeist: A noisy or mischievous ghost.
More specific terms:
* Revenant: A ghost that returns from the dead.
* Doppelgänger: A ghostly double of a living person.
* Haunt: A ghost that is said to inhabit a particular place.
* Entity: A being, especially one that is not easily defined or understood.
* Ethereal: A ghostly or supernatural being that seems to be from another world.
Informal terms:
* Boogeyman: A mythical creature that is used to scare children.
* Spook: A ghost or other supernatural being.
* Ghoul: A creature that is said to feed on the flesh of the dead.
The best word to use depends on the specific context.