An airlock is a device which permits the passage of objects, people, etc between a pressure vessel and its surrounding space while minimising the change of pressure in the vessel. It consists of two airtight doors which are never open simultaneously. A small volume of high pressure air, equal to the volume of the chamber between the doors, is reduced to the lower pressure space each time the airlock is operated.
In cold climates, two doors arranged in an airlock configuration are common in entrance ways to buildings. While not airtight, the double-doors minimize the direct flow of air in or out of the building, just like a real airlock. Revolving doors may be used for the same purpose.
The same principle, used with water, is found in the locks used on canals.
An airlock can also be used in brewing and is a simple 'S' shaped piece of plastic tubing that is half filled with water. When fitted into the top of a carboy by a rubber stopper the airlock allows waste carbon dioxide to escape while preventing outside air from bringing in bacteria that may spoil the batch. The positive pressure of the carbon dioxide inside of the carboy pushes out individual bubbles of carbon dioxide through the water and out to the air.