Lightness is a visual perception of the luminance of an object. It is often judged relative to a similarly lit object. In colorimetry and color appearance models, lightness is a prediction of how an illuminated color will appear to a standard observer. While luminance is a linear measurement of light, lightness is a linear prediction of the human perception of that light.
This distinction is meaningful because human vision's lightness perception is non-linear relative to light. Doubling the quantity of light does not result in a doubling in perceived lightness, only a modest increase.
The symbol for perceptual lightness is usually either as used in CIECAM02 or as used in CIELAB and CIELUV. ("Lstar") is not to be confused with as used for luminance. In some color ordering systems such as Munsell, Lightness is referenced as value.