Plain English Meaning
It is the control the pilot sets to tell the airplane how thick the air inside the cabin should feel, and how quickly it should change as the aircraft goes up or down.
Full Definition
A cockpit-adjustable device that regulates cabin pressure by commanding the outflow valve to release more or less air from the cabin, maintaining a selected cabin altitude and rate of change as the aircraft climbs, cruises, and descends.
Context Anchor
Seen in pressurized aircraft during preflight setup, climb, cruise, descent, and any discussion of cabin pressure control.