How close a colour is to white or black.
In the HSL colour model, lightness is the average of the maximum and minimum RGB channel values, expressed as a percentage from 0% (black) to 100% (white).
Lightness and value (HSV) are calculated differently and can diverge for highly saturated colours.
A pastel pink has high lightness; a deep burgundy has low lightness — both can share the same hue.
Lightness contrast between two colours is one of the main levers behind whether a pairing reads as subtle or bold.