A sealed, airtight chamber, such as in a manned spacecraft or submarine, used to provide access to and from the sealed area without allowing air out or water in.
A vestibule, foyer or entranceway with doors to the exterior on one end and doors to the interior on the other, functioning to keep indoor and outdoor air, humidity, and air temperatures separate.
A means of passing through a wall or floor, having a hatch (especially on a ship); a doorway with a hatch rather than a door.