Basically, the amount of light reflecting from a surface.
In printing, brightness is the amount of darkness of lightness of a printed image, described apart from the hue or saturation of the image. Brightness is typically a function of the reflectance characteristics of the paper being utilized, brightness being an optical property of paper that describes its reflectance of blue light, typically that at a wavelength of 457 nanometers, which is the wavelength at which the yellowing of paper is most easily gauged.