Airlock

For the episode of the Doctor Who serial Galaxy 4 entitled "Air Lock", see here.

Airlock for the Tiger Moth. (HOMEVID: Shakedown: Return of the Sontarans)

An airlock was a device that permitted the passage of people and objects between two drastically different environments, such as between a pressurised space and a vacuum or between an underwater and dry environment. The lock usually consisted of two airtight doors in series which did not open simultaneously. This allowed the intermediate chamber to change from one environment to the other.

References

The Aridians' city had airlocks. After the seas dried up, the Aridians would entomb the Mire Beasts by blowing up the airlocks. (TV: The Chase)

The Drahvin leader Maaga tried to kill Steven Taylor by leaving him in a de-pressurised airlock. (TV: Galaxy 4)

To prevent Kirksen from blackmailing the First Doctor into returning the ship to Kembel, and the Daleks, Katarina sacrificed herself by opening the airlock door on Mavic Chen's spaceship, Spar, expelling Kirksen and herself into the vacuum of space and killing them both almost instantly. (TV: The Daleks' Master Plan)

Varga allowed the Second Doctor to enter his spacecraft's airlock, then de-pressurised it until the Doctor agreed to answer his questions. (TV: The Ice Warriors)

The Doctor's TARDIS had an airlock which opened on the roof of the police box. (COMIC: The Underwater Robot, Peril at 60 Fathoms) However, on another occasion the Doctor claimed that the TARDIS didn't have an airlock. (PROSE: When Starlight Grows Cold)

The Kleptons' underwater city had an airlock through which they entered and exited in their flying machines. (COMIC: The Klepton Parasites)