Lightness
The measured or perceived amount of light in a colour, the L in HSL and Lab.
Also: luminance, brightness
Lightness says where a colour sits between black and white. Colour systems give it a number, usually from 0 to 100. The terms are easy to confuse. Luminance is a physical measure of light from a surface. Brightness is how strong that light appears to a viewer. Lightness is brightness judged relative to a white object in the same scene, which is why a sheet of paper looks white in both sun and shade. The eye is more sensitive to green light than to blue, so a pure green looks lighter than a pure blue.