Most people unfortunately don't know the difference between soul and spirit and use these terms interchangeably when they are two completely different things.

Ghosts are souls that failed to move on in the afterlife and are stuck (until they decide to move on) in the lower astral plane closest to Earth for whatever reason. Usually though these souls have a very material nature and cling to something or someone here in 3-D.

Its more appropriate to term them wayward souls, lost souls, warded souls or apparitions.