Plain English Meaning
The altitude your altimeter shows when you set it to the standard pressure value of 29.92. It gives every aircraft a shared reference point, regardless of the actual local pressure.
Full Definition
The altitude indicated when an altimeter's barometric setting is adjusted to 29.92 inches of mercury (the standard sea-level pressure). It represents height above the standard datum plane and is used as a common reference for aircraft performance calculations and for flight at higher altitudes.
Context Anchor
Pilots see pressure altitude in performance planning, especially when checking , , and capability.