Brightness
Brightness is an attribute of visual perception in which a source appears to emit a given amount of light."Brightness" was formerly used as a synonym for the photometric term luminance and (incorrectly) for the radiometric term radiance. "Brightness" should now be used only for nonquantitative references to physiological sensations and perceptions of light.
Source: from Federal Standard 1037C
With regard to stars etc. see apparent magnitude, absolute magnitude.
