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

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 more brighter to the human eye 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.