A poltergeist is a kind of ghost that does not just appear and look at you. It bangs, knocks, throws cups, slams doors, and makes a room feel as if someone angry is in it. People use the word when the scare is physical: things move, noises come from empty space. You would not usually call a quiet, see-through figure a poltergeist. English borrowed the whole German word and kept it. It belongs to ghost stories, films, and newspaper write-ups of “a haunted house,” not to calm scientific talk — though actually, reporters also stretch it for any unexplained bang in an old building, even when nobody really believes a ghost is there. Older folklore treats it as a type of spirit; everyday English treats it as a spooky story-label.