A 0–100 figure for how much visible light a paint or surface reflects, used in decorating and building design.
Essentially the luminance factor Y of the surface under a stated illuminant, expressed as a percentage. It is used for daylighting calculations and for visual-contrast guidance between surfaces such as walls, floors and doors.
LRV is not the same as the WCAG relative luminance of a colour's HEX value, and a HEX value cannot tell you a real paint's LRV.
A white ceiling paint might be around LRV 85; a charcoal around 10.
Room brightness and accessible contrast between surfaces are specified in LRV difference, not in colour names.