Plain English Meaning
An agreed-on 'zero point' set at the average level of the ocean's surface. Aviation altitudes are measured upward from this fixed reference, so every pilot and chart is using the same starting line.
Full Definition
The average height of the surface of the sea, calculated from continuous tide measurements over a long period, used as the standard reference datum for measuring altitude in aviation.
Context Anchor
Seen on aeronautical charts, airport information, altimeter settings, and instrument procedures when heights are given relative to .