"Absolute magnitude" is a term used to describe the brightness of a star or other celestial object. It measures how bright the object would appear if it were located at a standard distance from Earth. Full definition
«Surprisingly, this is one of the faintest satellites, with absolute magnitude of -0.8 in the optical waveband. (sciencedaily.com)
You first plot the cluster's main - sequence on a color - magnitude diagram with apparent magnitudes, not absolute magnitude. (astronomynotes.com)
Occasionally, at irregular intervals given by chance, in any type of galaxies, a supernova occurs: This is a star suddenly brightning to a high luminosity which may well outshine the whole galaxy; the maximal absolute magnitude of a supernova may well reach -19 to -20 magnitudes. (maa.clell.de)