Plain English Meaning
The altitude your altimeter shows based on air pressure. As you climb, air pressure drops, and the altimeter converts that pressure change into a height reading.
Full Definition
An altitude derived from atmospheric pressure, measured by a barometric altimeter and referenced to a selected pressure setting (such as the local altimeter setting or the standard 29.92 inHg). It represents the aircraft's height above that pressure reference, not its true height above the ground or above sea level under all conditions.
Context Anchor
Seen in instrument flying, altimeter use, air data displays, and discussions where pressure-based altitude is one of the inputs.