Official Aviation Definition
Cabin altitude is the apparent atmospheric pressure inside the fuselage of a pressurized aircraft, expressed in terms of equivalent altitude above mean sea level. While a jet cruises at 38,000 feet, the environmental control system typically pressurizes the cabin to a comfortable 6,000 to 8,000 feet equivalent.
In Simple Words
How pressurized the inside of the passenger cabin is. Even when the plane is at 35,000 feet, your lungs feel like you are on top of an 8,000-foot mountain.
Real-World Flight Example
Modern composite airliners like the Boeing 787 and Airbus A350 maintain a cabin altitude of approximately 6,000 feet for improved passenger comfort.