Plain English Meaning

Air pressure outside that's higher or lower than the agreed-upon 'normal' value. When this happens, your altimeter will read wrong unless you update its setting to match the actual pressure where you are.

Full Definition

Atmospheric pressure at a given location that differs from the standard sea level value of 29.92 inches of mercury (1013.2 millibars) at 15°C. When pressure is nonstandard, an altimeter set to 29.92 will not display the aircraft's true height above sea level, so the altimeter must be reset to the current local altimeter setting to read correctly.

Context Anchor

Seen in altimeter and pitot/static instrument discussions, especially when why pilots set the local altimeter setting , , and .