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.
Plain English
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.
Context Anchor
Seen in altimeter and pitot/static instrument discussions, especially when learning why pilots set the local altimeter setting before takeoff, approach, and landing.
Derivation
Nonstandard' simply means 'not the standard.' The 'standard' here is the International Standard Atmosphere, a fixed reference of 29.92 inHg at sea level and 15°C. Real air rarely matches this exactly, so most of the time the air around you is technically nonstandard.
Why Pilots Care
Failure to correct for nonstandard pressure produces altitude errors that can result in controlled flight into terrain or airspace violations.
Analogy
Think of the altimeter like a scale that must be zeroed before use. If the pressure setting is wrong, the instrument still moves, but its starting point is off.
Grounding Statement
On a low-pressure day, an altimeter set to the wrong pressure can make the airplane’s altitude look higher than it really is.
Intuition Check
Nonstandard does not mean unusual, unsafe, or broken. It means the actual air pressure is different from the standard reference value used by the altimeter.
Example Sentence 1
Because of nonstandard pressure along the route, the pilot updated the altimeter setting at each new ATIS frequency.
Example Sentence 2
Nonstandard pressure at higher elevations required frequent altimeter resets to maintain accurate terrain clearance.