Brightness
Brightness is an attribute of visual perception in which a source appears to emit a given amount of light. In other words: Brightness is the perception elicited by the luminance of a visual target.
"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
Note, that the same target luminance can elicit different perceptions of brightness in different contexts. (See, e.g. White's illusion and Wertheimer-Benary illusion)
In the RGB color space, brightness can be thought of as the arithmetic mean μ of the R, G, and B color coordinates (although some of the 3 components make the light seem brighter than others, which, again, may be compensated by some display systems automatically):
Brightness is also a color coordinate in HSB color space.
With regard to stars etc. see apparent magnitude, absolute magnitude.