Definition
The altimeter setting that causes the altimeter to read the airport's elevation above mean sea level when the aircraft is on the ground at that airport. It is the local barometric pressure adjusted to sea level, expressed in inches of mercury (in Hg) in the United States or hectopascals (hPa) in most other countries. When set in the altimeter's Kollsman window, QNH makes the instrument display altitude referenced to mean sea level (MSL).
Plain English
QNH is the pressure number you dial into the altimeter so it shows your height above sea level. When you're sitting on the ramp, setting QNH makes the altimeter read the field's published elevation.
Context Anchor
Pilots encounter QNH in altimeter-setting procedures, especially in international flying and in aviation weather or air traffic control information outside the United States.
Derivation
QNH comes from the Q-code system developed in the early 1900s for radio operators, where each three-letter code starting with Q stood for a complete question or answer. The letters themselves don't spell anything; QNH was simply the assigned code for the question 'What should I set on my altimeter so it reads field elevation on landing?' Knowing this prevents the trap of trying to decode the letters.
Why Pilots Care
Correct QNH prevents altitude errors that could lead to terrain conflicts or airspace violations.
Intuition Check
Do not treat QNH as a normal word or as initials for a phrase. It is a standard radio code for a specific altimeter pressure setting.
Example Sentence 1
Approach passed us the latest QNH of 30.12, and we set it in both altimeters before starting the descent.
Example Sentence 2
With the local QNH set, the altimeter read field elevation when we were parked on the ramp.