THING YOU OPEN › the part of a building, room, vehicle, or piece of furniture that you open or close to get inside it or out of it: Please shut the door behind you. I can't open the door. There's someone at the door.
SPACE › the space in a wall where you enter a building or room: He led us through the door to the rear of the building.
behind closed doors › privately and not in public: Most of the deals were done behind closed doors.
two/three, etc doors away › in a place that is two/three, etc houses away: We live just a few doors away from the Smiths. → See also trap door