No one really knows what it is, a ghost.
The easiest and most common definition is an earthbound spirit of someone who has died. But that really doesn’t take into account about ninety percent of what people experience.
Some people believe that it’s time, overlapping. We see and hear things that happened in houses and rooms from long before, as though a place can have a memory.
Others add on this this with the notion that specifically traumatic or negative acts can create an echo of an event or even a person, superimposing it on time itself like a double exposure on a strip of film. And, in time, it fades.
Other things are much more active, but less coherent: Wandering shadows, balls of light, poltergeists, phantom smells and brief moments of physical contact . . . who knows?
And some people believe (even smart people, like G.K. Chesterton) that the whole of the spirit world is a masquerade, a sham. Ouija boards, seances, channeling . . . all of it that we come into contact with is merely a supernatural (perhaps supranatural) element having some fun with us, trying to lead us astray, or into despair, or to cripple us with fear.
I used to believe that Chesterton was right. I’ve since modified my opinions somewhat. I do still believe that there’s a lot of imposters talking to unwitting people through Ouija boards and mediums . . . but I think there are more things in heaven and earth, Horatio (to counterfeit a phrase).
It really isn’t important, when it comes right down to it. Whether they’re wandering spirit, persistent temporal echo, spiritual cartoon, or malicious demon . . . the ghosts have had their time, they’re trespassers and they have no right to intrude.
They should be thrown out. And they can be.
You just gotta know what you’re dealing with and act accordingly.
Which is, perhaps, a conversation for another time.