Plain English Meaning
An NDB navigation beacon that is physically on the airport grounds. Flying over it tells you that you have reached the runway area, so it serves as the point where you either see the runway and land, or go missed.
Full Definition
A non-directional radio beacon (NDB) located on the airport itself, used as the final approach fix for an NDB instrument approach. Because the beacon sits on the field, station passage over the NDB marks arrival at the missed approach point, and no separate final approach fix is published off-airport.
Context Anchor
Seen on NDB approach and in descriptions.