From AMS Glossary
(Also called standard visibility, standard visual range.) An empirically consistent measure of the visual range of a target; a concept developed to eliminate from consideration the threshold contrast and adaptation luminance, both of which vary from observer to observer.
The meteorological range is the distance V′ in the black target form of the visual-range formula,
when ε, the threshold contrast, is set equal to 0.02. Thus, V′ is a function only of the extinction coefficient σ of the atmosphere at the time and place in question.