Let's just list all the possibilities popularly mentioned:
Ghosts are the spirits (souls) of the dead.
Ghosts are other dimensional beings.
Ghosts are recorded events from the past.
Ghosts are time jumps by those in the living world.
Ghosts are psychic projections by the living.
Ghosts are an issue with human psychology/senses.
Ghosts are angels and noncorporeal beings who were never in human form.
Ghosts are misinterpretations of explainable occurrences.
Ghosts are life forms from our own world that we don't have proper senses to detect except in the right conditions.
Ghosts are alien beings.
So, where do you stand?