An airlock is a device which permits the passage of people and objects between a pressure vessel and its surroundings while minimizing the change of pressure in the vessel and loss of air from it. The lock consists of a small chamber with two airtight doors in series which do not open simultaneously.
An airlock may be used for passage between environments of different gases rather than different pressures, to minimize or prevent the gases from mixing.
An airlock may also be used underwater to allow passage between an air environment in a pressure vessel and a water environment outside, in which case the airlock can contain air or water. This is called a floodable airlock or an underwater airlock, and is used to prevent water from entering a submersible vessel or an underwater habitat.