1 Division between one storey and another. In the UK ground-, first-, second-, etc., described the storeys, but in many countries ‘first-floor’ means the ‘ground-floor’, etc.
2 Set of rooms and landings, on the same level, i.e. in one storey.
3 Layer of boards, etc., on which people tread, and i.e. the surface of the floor in the room.