Definition
QNH is the local station pressure, corrected to mean sea level using the standard atmosphere, that a pilot dials into the altimeter's Kollsman window so the instrument indicates true elevation above sea level. When QNH is set correctly, an aircraft sitting on the airport ramp will display the published field elevation, and in flight the altimeter will indicate altitude above mean sea level (MSL) referenced to current local pressure conditions.
Plain English
QNH is the local pressure number you put into your altimeter so it reads the airport's actual elevation when you're on the ground, and your real height above sea level when you're flying.
Context Anchor
You encounter QNH in altimeter-setting discussions, instrument procedures, weather reports, and radio calls where a local pressure setting is given for accurate altitude readings.
Derivation
QNH is one of the international Q-codes, a system of three-letter brevity codes developed in early radio communication for ships and later adopted by aviation. The codes are arbitrary identifiers, not abbreviations of words. QNH historically distinguished this specific pressure setting from QFE (pressure that makes the altimeter read zero on the field) and QNE (the standard 29.92 inHg / 1013.25 hPa setting used at high altitudes).
Why Pilots Care
An incorrect QNH produces altitude errors that can lead to controlled flight into terrain or loss of required vertical separation from other aircraft.
Grounding Statement
QNH connects the current local air pressure to the altimeter so the instrument shows altitude from the same sea-level reference used on charts and procedures.
Intuition Check
QNH is not simply the actual air pressure at the airport. It is the pressure setting adjusted so the altimeter reads elevation above sea level correctly.
Example Sentence 1
Approach passed us the current QNH of 30.05, so I set it in the Kollsman window and confirmed the altimeter showed field elevation as we crossed the threshold.
Example Sentence 2
Approach control issued the destination QNH so the crew could maintain the assigned altitude on the ILS.