An airlock, air-lock or air lock, often abbreviated to just lock, is a compartment with doors which can be sealed against pressure which permits the passage of people and objects between environments of differing pressure or atmospheric composition while minimizing the change of pressure in the adjoining spaces and mixing of environments. The lock consists of a relatively 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 or different pressures, or both, to minimize pressure loss 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 the 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.

Air-locks are used in space flight, saturation diving accommodation, hyperbaric chambers, submarines, some underwater habitats, cleanrooms, and pressurised caissons and tunnels. Depending on the pressure difference, when people use an airlock to transfer from higher to lower pressure, it may be necessary to follow a decompression schedule to avoid decompression sickness. A similar arrangement may be used for access to airtight clean spaces, contaminated spaces, or unbreathable atmospheres without necessarily having a pressure difference. In these cases a decontamination procedure and flushing takes the place of pressure changes.

The procedure of entering an air-lock, sealing it, equalising the pressure, and passing through the inner door, is known as locking in. Locking out is to unseal the outer door after equalising pressure and exit the lock compartment to the ambient environment. Locking on and off refer to transfer under pressure where the two chambers are physically connected or disconnected prior to equalizing the pressure and locking in or out.