What is a ghost?

Answer 1

Answers (quite a few)
Ghosts are a paranormal state, the soul or spirit of someone who has died, who can't "pass over" , for some reason. Either they left something very special that they are protecting, or they haven't gotten to the state where they choose Heaven or Hell.
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A ghost is the supposed spirit of somebody who has died, believed to appear as a shadowy form or to cause sounds, the movement of objects, or a frightening atmosphere in a certain place. Or someone who has died before their expiration date and their spiritual form has not found peace, so it still walks the earth.
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A ghost is an alleged non-corporeal manifestation of a dead person (or, rarely, an animal or vehicle). It is often thought to be a manifestation of the spirit or soul of a person which has remained on Earth after death. According to some beliefs, a ghost may be the personality of a person after his or her death, and not tied directly to the soul or spirit. Every culture in the world carries stories about ghosts, but they vary across time and place, with disagreements both as to what ghosts are and whether they are just figments of imagination or a part of reality.
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A Ghost is a Dead person that didn't move on. Some say they have unfinished business.
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It is said by some ancient Eastern religions that we have 3 bodies or "coverings"; a gross physical body, a luminous subtle astral body, and an even more subtle casual body.
When a sentient being dies, it looses its gross physical body. The subtle astral body and casual body are left. What we are seeing as a "ghost" is the subtle astral body which is still present. The casual body is much harder to see.
Since we are used to inhabiting the gross physical body, we see the astral body as less real and more ethereal. However, if we happen to lose our physical body, as in death, the astral body would seem more real.
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A ghost is a spirit who isn't using or inhabiting a physical body. -----------------

A ghost is potentially the result of a person who has passed away, and their cognitive and subconscious mind has been shared with others throughout their lives. When they passed, certain physical references trigger the prominence of that deceased person's thought in the living. That is why ghosts can not be photographed reliably; they exist within our minds.

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Ghosts are supposedly the manifestation of the souls of the dead in the realm of the living. Often events for which there is no ready explanation are attributed to ghosts (strange sights/noises, technical interference with cameras and the like). And though almost all cultures have the concept of ghosts, there has never been any sound evidence of their existence.
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Ghosts are paranormal spirits that sometimes can't find their way home or still have a deed to accomplish. There are 3 types of ghosts, poltergeist, residual, and intelligent (sorry for the spelling). Poltergeists are ghosts that either, got sent back from Hell or didn't want to give up torturing people, residual spirits are ones that play over like a tape recorder. Intelligent are the ones who touch people, whisper, or move things.
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Ghosts are imaginary forms people create, to be able to handle grief, or to draw attention to themselves, or even to make money out of people's hopes that their loved ones aren't just gone forever.. which of course they are.
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They won't harm you - they just need help from us because they couldn't go to heaven.
An Additional Point Many people have been saying that they can't hurt you or they are imaginary, but they can hurt you and there have been many reports. And I wouldn't say ghosts, I would say "spirits" It's a much better term for them and really they are spirits that are trapped on earth. There are also poltergeists, demons, etc. These are the unfriendly things that can harm you. The official definition for the word ghost is "an apparition of a dead person that is believed to appear or become manifest to the living, typically as a nebulous image."