Plain English Meaning
The official height reference point at an airport that altimeter settings are based on. It's the spot the airport uses as its 'official elevation' for pressure and altitude purposes.
Full Definition
The fixed reference point at an airport from which altimeter settings are determined, typically the airport's official elevation. When the local altimeter setting (QNH) is set in the Kollsman window, the altimeter reads height above mean sea level; when the airport's pressure setting (QFE) is set, the altimeter reads zero on the ground at the airport altimeter datum.
Context Anchor
Seen in instrument procedure discussions of QFE and altitude above ground, especially when comparing height above the airport with altitude above sea level.