Cabin pressurization is the active pumping of compressed air into an aircraft cabin when flying at altitude to maintain a safe and comfortable environment for crew and passengers in the low outside atmospheric pressure.

Pressurization is essential over 3,000 metres (9,800 ft) to prevent crew and passengers from becoming unconscious through the lack of oxygen (hypoxia) in the thin air above that altitude. Pressurization also removes or alleviates a number of other adverse physiological effects of altitude (see below) and increases passenger comfort generally.