would like to request for your opinion on "What is ghost?"

Normally we would think that ghost is from the "soul" of a living thing. If that is
the case, then the world would be full of ghosts - and certainly this does not look
so.

I would think that someone becomes a ghost when he/she/it has something undone
when he/she/it dies. So, he/she/it is trying to have the outstanding thing done
before moving to the right world he/she/it should go to.

I am from Malaysia and I have heard many dreadful stories on ghosts disturbing
people. And recently I came across a new friend who said that he could see
ghosts since he was a small boy.

Please enlighten me on the above, and thank you.