This is a sticky subject. There is certainly a dictionary definition of 'ghost', more than one I expect, but it has to be a subjective opinion. Some who have experience of a ghostly apparition have assumed it to be the disembodied spirit of a dead person. This seems to be the most common view. But there are reports of ghosts, phantasms, of living people too. Uninformed opinion often attributes the sighting of an apparition as being 'all in one's head'. One has to agree with that uninformed assessment, because all we ever experience is created within our heads - from the various data arriving from the outside world - collected by our various sense organs and maybe in other ways, to be translated and (supposedly) accurately reconstituted as a combination visual, auditory, olfactory, etc. experience obtained from our environment.
At the same time, to ask the question 'Do you believe in ghosts!' is to me, and surely should be to everyone, no more meaningful, or useful in getting to grips with the phenomenon, than asking someone, ‘do you believe anyone wore mauve and yellow woolen socks in Milton Keynes yesterday!' Just another crackpot opinion, of course.