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An airlock is a device which permits the passage of objects, people, and the like, between a pressure vessel and its surrounding space while minimizing the change of pressure—and loss of air—in the vessel. The lock consists of two airtight doors in series which do not open simultaneously.

Before opening either door, the air pressure of the airlock (volume between the doors) is equalized with that of the environment behind the next door to open. A gradual pressure transition minimizes air temperature fluctuations (see Boyle's law), which helps reduce fogging and condensation, decreases stresses on air seals and allows safe verification of pressure suit and space suit operation. In situations where an unprotected human is moving from one pressure to another—such as a scuba diver—the slow pressure change helps with internal air cavity equalization, and is critical in preventing decompression sickness.

A four-door airlock (with, therefore, three interior chambers) was proposed by science fiction writer H. Beam Piper in his novel Uller Uprising. In the proposal, the atmosphere inside the structure was human-breathable, while the outside atmosphere was highly toxic. Only one door of the airlock was to be opened at a time, and the middle chamber of the three would always contain a vacuum. Piper implied that this design would minimize traces of the exterior atmosphere traveling inside.

Airlocks are used in

• submarines to permit frogmen or other divers to exit and enter underwater. Torpedo tubes and escape trunks in submarines also act as a form of airlock.
• spacecraft, to prevent pressure loss when exiting or entering the craft.
• hazardous environments, such as nuclear reactors and some biochemical laboratories.
• pressurized domes such as the USF Sun Dome

In cold climates, two doors arranged in an airlock configuration are common in building entrances. While not airtight, the double doors minimize the loss of heated air from the building. Revolving doors may be used for the same purpose.

The same principle is employed in the waterway locks used on canals.