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An airlock aboard the NX-01
An airlock on an Intrepid-class vessel
One of Deep Space 9's characteristic airlocks
The interior of an airlock aboard Deep Space 9
An airlock is the point of entry to a space station or starship which allows ingress and egress to empty space or another vessel which has docked at the portal.
Typically airlocks are located on the port or starboard sides of starships rather than in a ventral or dorsal position which are usually reserved for the presence of cargo bays or shuttlebays.
The airlocks on Deep Space 9 had built-in weapon detectors capable of detecting military technology carried by persons boarding the station from docked starships. (DS9: "Captive Pursuit")
A frequent threat in space travel was to be "thrown out" of an airlock into the vacuum of space.
In 2153 Jonathan Archer tortured an Osaarian for information regarding the Xindi superweapon by placing him in an airlock and depressurizing it, threatening the Osaarian with suffocation if he did not provide the desired information. (ENT: "Anomaly")
In 2369 three Federation passengers were trapped in an airlock for about an hour. The repair crew of Deep Space 9 tried to free them. (DS9: "Babel")
Airlocks were sealed and turbolifts were secured during a security alert. (DS9: "Dax")
Jake Sisko and Nog frequently observed Bajoran girls who left Bajoran transports from airlock three. (DS9: "Move Along Home")
Krax and Rom attempted to kill Quark in 2369 by decompressing an airlock. Quark previously threatened Rom he would toss him out of the nearest airlock because Rom acted not as the Rules of Acquisition said. (DS9: "The Nagus")
Security chief Odo was able to pass a closed airlock by morphing himself into a liquid substance. (DS9: "The Nagus")
Quark once told Odo that everyone would be scared when the Starfleet officers greeted them at the airlock when he talked about Croden who tried to avoid contact with Commander Sisko. (DS9: "Vortex")
In 2369 the Bajoran Kai Opaka arrived at airlock fourteen. Julian Bashir informed Commander Sisko about this surprising visit. (DS9: "Battle Lines")
When Major Kira evacuated the lower pylon one she left a message over com that all personnel had report to the outer ring airlock. (DS9: "If Wishes Were Horses")
Chief O'Brien and Commander Sisko materialized in airlock six corridor instead on board the Valerian transport when they beamed off ops in 2369. (DS9: "Dramatis Personae")
While investigating the murder of Ensign Aquino in runabout pad C, Neela mentioned that the airlock system was unattached. (DS9: "In the Hands of the Prophets")
In 2370, the USS Yellowstone docked at Deep Space 9's airlock 14 to drop off Ensign Melora Pazlar. (DS9: "Melora")
Elim Garak attempted to open an airlock on Deep Space 9 following an attack of claustrophobia in early 2375. (DS9: "Afterimage")
Many of the doors on the alternate reality's USS Enterprise, launched in 2258, were airlocks, including the starboard entrance to the bridge. (Star Trek)