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A poltergeist (from German poltern "to make noise" and geist "ghost" or "spirit") is an entity capable of interacting with the physical plane via noises and moving objects, sometimes to the point of launching them across rooms. They're also said to be capable of hitting, pinching, biting, scratching, and tripping people, among other things. Poltergeists seem to tend to haunt a particular person.
One prominent theory to explain poltergeist activity is that most if not all of it is the psychic manifestation of a residential teenager's emotions. This explains why the activity seems to follow a particular person. In this sense, a poltergeist would be like a powerful servitor energetically fed by those emotions.