Technical term used in color science, design, and physics to describe the perceived luminance or lightness of a color or surface.
The three attributes of color are hue, saturation, and brightness.
This color has high lightness, so text is hard to read when used as a background.
Compound of 明 (bright, light) and 度 (degree, measure), literally 'degree of brightness'. A modern technical coinage for the concept of lightness in color theory.