Plain English Meaning
A way to confirm your two onboard navigation receivers are accurate by tuning them both to the same ground station and checking that they agree within 4 degrees of each other.
Full Definition
An accuracy check of an aircraft's VOR navigation equipment performed by tuning both VOR receivers to the same VOR station, centering the course deviation indicator on each, and comparing the two indicated bearings. The maximum permissible variation between the two indicated bearings is 4 degrees.
Context Anchor
Used when checking whether installed VOR navigation equipment is accurate enough for instrument flight, especially before relying on it in the airplane or logging the required VOR check.