Plain English Meaning

It is the standard pressure setting pilots dial into the altimeter when flying high enough to use flight levels. Setting this value means every aircraft above the transition altitude is measuring height from the same agreed reference, so they can be safely separated even if the actual local pressure is different.

Full Definition

The international standard sea-level atmospheric pressure value used as the reference setting for altimeters when flying at flight levels. 1013.2 hectopascals (hPa) and 1013.2 millibars (mb) are numerically identical units of pressure; the hectopascal is the modern SI unit and has replaced the millibar in most aviation publications, though the two terms are used interchangeably. This value corresponds to 29.92 inches of mercury (inHg).