Plain English Meaning
A gauge that shows how high the inside of the cabin 'feels' in terms of air pressure, even when the aircraft is flying much higher than that.
Full Definition
An instrument in a pressurized aircraft that displays the effective pressure altitude inside the cabin, allowing the crew to monitor the cabin pressurization environment that the occupants are actually breathing.
Context Anchor
Seen on the pressurization panel or pressurization display in aircraft that maintain cabin pressure during flight.
Derivation
From 'cabin' (the enclosed occupant space) and '' (an instrument that measures by sensing air pressure). The name reflects what it does: it measures altitude, but for the cabin instead of the aircraft.