Ghost

from the Buffyverse Wiki - a free Buffy and Angel fan community

Ghost was a term employed to refer to the disembodied spirits of deceased humans.

There seem to be different kinds of disembodied spirits other than ghosts. When Spike was rendered incorporeal following the Battle at the Hellmouth, Fred's scans indicate that Spike is technically not a ghost: he lacked ectoplasm, which would make him invisible to human eyes, radiated body heat, whereas ghosts absorb heat and make the area around them a few degrees colder, and had detectable brainwaves.