Plain English Meaning
A painted spot on the ground at an airport that tells you exactly where to park so you can test whether your aircraft's navigation receiver is reading correctly. If you sit on this spot, the receiver should show a specific bearing to the nearby navigation station; if it doesn't match within the allowed tolerance, the receiver isn't accurate enough to use for navigation.
Full Definition
A painted circle on an airport surface, typically on a taxiway or ramp, marking a precise location where a pilot can park the aircraft and check the accuracy of the onboard VOR navigation receiver against a published bearing and distance from a nearby . The marking is a solid white circle, about 6 feet in , with a white arrow inside pointing toward the VOR station. The exact location, the published , and the tolerance are listed in the .