Official Aviation Definition
Runway Visual Range (RVR) is an electronically measured meteorological parameter representing the horizontal distance over which a pilot on the runway centerline can see the runway surface markings or lights delineating the runway. It is measured using transmissometers situated along the runway.
In Simple Words
The exact distance in feet or meters that a pilot can see runway lights through fog when touching down.
Real-World Flight Example
Low-visibility operations were initiated when the touchdown zone RVR dropped below 600 meters.