An opening in the wall or roof of a building, for admitting light or air and allowing people to see out; esp. such an opening fitted with a frame containing a pane or panes of glass (or a similar transparent substance); the glazed frame intended to fit such an opening, sometimes with hinges, a sliding mechanism, etc., so that it may be opened or closed. Also: a similar opening in the side of a ship, train, car, or other vehicle.