A cylindrical module in a space shuttle or space station used by astronauts to exit to space and return. When beginning extravehicular activity (EVA), astronauts enter the airlock and close the inside hatch. After checking that the spacesuits operate perfectly, they undergo prebreathing and then vent air out of the airlock. They then open the outer hatch and each pulls in a safety tether from outside and attaches it before entering space. After an EVA, the astronauts float into the airlock, remove the tethers and hook them outside, close the outer hatch, repressurize the airlock, and open the inner hatch to return to the main spacecraft.

In the airlock is an airlock adapter plate, a framed fixture used to store spacesuits. Astronauts normally use the area in front of the fixture to put on and remove their spacesuits. The floor of the airlock is a foot restraint platform, which has a grid design into which a triangular metal piece on the sole of each shoe can be inserted.