The term “poltergeist” is from German and means “noisy ghost”. It was originally used to describe ghosts that can create physical disturbances such as moving objects and making noises. In order to do this it is necessary for an entity that exists only in spiritual form to be able to affect earthly matter. To be clear, it is only marginally easier for a ghost to move earthly matter than it is for a person to move an object without touching it. Not all ghosts are capable of psychokinetic effects. Some ghosts can make their presence known to living people only by producing mental images and hallucinations. The ability for any person or spiritual entity to move earthly matter without directly or indirectly touching it is called Psychokinesis, or PK for short.
Psychokinesis is a facility shown to be a property of ordinary living human beings. In most people, it is generally a very weak property. However, in certain adolescents and in rare psychic adults, it can be strong enough to move observable objects without physical contact. Because of this, it can be almost impossible to tell the difference between psychokinetic effects caused by the subconscious mind of the living agent and the activity of a spiritual presence.
The term poltergeist was originally created by a nineteenth century parapsychologist named René Sudre. Sudre believed that psi facilities were innate properties of living people. He did NOT believe in a spiritual world, or that spirits were responsible for the undeniable evidence of violent or noisy “hauntings”. He believed that all psychokinetic phenomena normally attributed to ghosts were, in fact, created by human observers who unconsciously produce these disturbances without being aware that they are doing it themselves.
Today, skeptical parapsychologists use the term “poltergeist” as a way to evade the question of a spiritual cause for psychokinetic phenomena like the production of sounds that can be recorded, the levitation of objects, the slamming of doors and the other violent activity seen in hauntings and possession cases. That way, a skeptical parapsychologist can avoid the stigma of being called a “ghost hunter”. They want to to be seen by a materialistic public as scientists rather than dreamy mystics, so some of them just assume that all paranormal phenomena are caused by living humans, either through PK or fraud. Others will admit that some of the phenomena are difficult or impossible to explain but publicly maintain their skepticism anyways.
Today, it is frankly, more “profitable” for a professional parapsychologist to be skeptical of spiritual explanations for apparently ghostly manifestations. They are more likely to be hired into academic positions by skeptical university administrators than parapsychologists that admit to a belief in spiritual entities. Thus they attribute the paranormal movement of objects to a subconscious desire on the part of, say, a distressed person in a household to “lash out” at the persons causing their distress. For all practical purposes, therefore, the difference between a ghost and a poltergeist is simply a matter of belief. If you believe in human psi but don’t believe in spirits, you call the manifestations a poltergeist. If you believe in Spirit, you call it a ghost or a spirit.
The major reasoning behind the belief that psychokinetic disturbances are strictly attributable to human-caused psychokinetic effects is that some cases, especially the most violent ones seem to follow a particular person from place to place instead of being tied to a particular location like most ghosts.
Of course, not all parapsychologists are skeptics, and a second definition of psychokinetic manifestations has emerged. It is used by parapsychologists who either believe in a spiritual realm, or at least do not rule out the possibility. For these scientists, the major differences between a ghost and a poltergeist is that ghosts are tied to a particular place and generally produce relatively mild psychokinetic phenomena while poltergeists tend to follow a particular person (called a poltergeist “agent”) from place to place. In either case, objects are thrown, noises are heard, faucets turn on without anyone in the room, and puddles appear on the floor without any indication of a leak.