Plain English Meaning
A controllable opening in the side of a pressurized aircraft that lets cabin air escape at just the right rate to keep the inside of the cabin at a comfortable, breathable pressure.
Full Definition
A valve in a pressurized aircraft that controls the rate at which conditioned cabin air is released from the fuselage to the outside atmosphere, thereby regulating cabin pressure. It works in concert with the air conditioning and pressurization system: air is forced in at a steady rate, and the outflow valve modulates how quickly that air leaves, holding the cabin at the scheduled pressure altitude.
Context Anchor
Seen in aircraft pressurization system descriptions, maintenance checks, and abnormal cabin pressure discussions.