Definition
A condition in which the actual atmospheric pressure differs from the standard sea level pressure of 29.92 inches of mercury (1013.2 hPa), causing a pressure altimeter to display an altitude that does not match the aircraft's true height above mean sea level unless the altimeter setting is adjusted to the current local pressure. When flying from an area of higher pressure into an area of lower pressure without resetting the altimeter, the instrument indicates an altitude higher than the aircraft is actually flying; when flying from lower pressure into higher pressure without resetting, it indicates an altitude lower than the aircraft is actually flying.
Plain English
The air pressure outside isn't always the textbook value the altimeter is built around. When it isn't, the altimeter reads wrong unless the pilot dials in the current pressure setting. If you fly into lower pressure without updating the setting, the altimeter overreads — you're lower than it shows. If you fly into higher pressure without updating, it underreads — you're higher than it shows.
Context Anchor
Seen when learning altimeter errors, setting the altimeter before flight, and updating the altimeter setting during cross-country or instrument flight.
Derivation
Nonstandard' simply means 'not matching the standard.' The standard here is the ICAO standard atmosphere, which assumes 29.92 inHg at sea level. Any pressure other than that is, by definition, nonstandard.
Why Pilots Care
Nonstandard pressure creates altitude errors that can place the aircraft lower or higher than the pilot believes, directly affecting terrain clearance and vertical separation.
Analogy
It is like a bathroom scale that has not been set to zero before you step on it. The scale still gives a number, but the number is offset because the starting point was wrong.
Grounding Statement
If a pilot flies into an area of lower pressure and does not reset the altimeter, the aircraft can be lower than the altimeter indicates.
Intuition Check
Do not assume “nonstandard pressure” means unusual or dangerous weather by itself. Here it means the actual air pressure does not match the pressure value the altimeter is using to calculate altitude.
Example Sentence 1
Approaching the destination, the pilot received a new altimeter setting from ATIS because nonstandard pressure in the area would have caused the altimeter to read several hundred feet off if left at the previous setting.
Example Sentence 2
Flying from high pressure into low pressure without adjustment caused the altimeter to read higher than the aircraft's actual altitude.