An airlock is a device which permits the passage of people and objects between the interior and exterior of a pressurized vessel without compromising the atmosphere aboard the vessel. It consists of a small compartment sealed by two airtight doors, only one of which can open at any given time to prevent loss of pressure. By entering the chamber, the occupants can move from one side to the other by equalizing the pressure in the direction they wish to move. Airlocks are a staple in the design of any spacefaring or submersible vessel.
Teal'c once used an airlock on an Asgard Bilskirnir-class ship to exit the vessel and place a shaped charge on the outer hull of the ship. (SG1: "Nemesis")
Frank Simmons, under the control of a Goa'uld, was sucked out of an airlock on the Prometheus. (SG1: "Prometheus") On the Daedalus, a Wraith computer virus used an airlock to kill Dr. Lindstrom, tricking him into entering it then venting the airlock into space. (SGA: "The Intruder")
Atlantis and other city-ships are an exception to the common idea of having airlocks on spacecraft, as the city's sole means of retaining air is its shield. However, it does possess an underwater Jumper bay which can be drained of water to allow the passengers to disembark. (SGA: "The Return, Part 2")
The Ancient vessel Destiny is equipped with several airlocks. Each shuttle has an individual airlock. However, the gate room and the surrounding areas do not have airlocks, so the crew must resort to passing through hull breaches by reducing the shield strength and depressurizing the surrounding area. Destiny's doors are airtight, limiting such decompression to a single hallway. The only personnel airlock shown thus far is one near the bow of the ship. A panel on the airlock lights up yellow when pressurized, and red when sealed. The airlock has no remote access, nor can it be opened from the outside, requiring someone to be physically present at the airlock to open and close it. Scott and Greer used this airlock to re-enter the ship when they were kept from using the hull breach near the gate room, aided by Eli Wallace. (SGU: "Divided", "Incursion, Part 2", "Intervention")