Plain English Meaning
An imaginary flat surface in the sky where the air pressure is set at the standard value of 29.92. Altitudes measured from this imaginary surface are called pressure altitudes.
Full Definition
A theoretical level in the atmosphere at which the air pressure is exactly 29.92 inches of mercury (1013.2 millibars) and the temperature is 15 degrees Celsius. It is the fixed reference used to measure pressure altitude.
Context Anchor
Seen in instrument flying, altimeter setting, and pressure altitude discussions.