An airlock is a device which permits the passage of people and objects between a pressure vessel

Pressure vessel

A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...

 and its surroundings while minimizing the change of pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 in the vessel and loss of air from it. The lock consists of a small chamber with two airtight

Hermetic seal

A hermetic seal is the quality of being airtight. In common usage, the term often implies being impervious to air or gas. When used technically, it is stated in conjunction with a specific test method and conditions of usage.-Etymology :...

 door

Door

A door is a movable structure used to open and close off an entrance, typically consisting of a panel that swings on hinges or that slides or rotates inside of a space....

s 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.