Airlock (noun)

1. A chamber or passage that separates the inside of a spacecraft, submarine, or other sealed vessel from the outside environment.
2. A mechanical device used in diving that prevents the loss of air from the lungs when changing pressure.
3. A device used to prevent the loss of air from a building or room to the outside environment.

Origin:

From air + lock, from the sense of a barrier or restriction in airflow.

Examples:

1. The astronaut entered the airlock and prepared for the spacewalk.
2. The airlock is used to equalize the pressure between the inside and the outside of the sub.
3. The airlock system is used to prevent the loss of air conditioning in the building.
4. The diver used the airlock to safely surface from the deep dive.
5. The airlock is the last barrier before entering the vacuum of space.