apparent magnitude

Short definition from Wiktionary

Noun

1. 1
   a numerical measure of the brightness of a star, planet etc.; a star with apparent magnitude 0 gives an illuminance of 2.54 microlux just outside the earth's atmosphere, or 82% of that (2.08 microlux) under clear skies, and every increase of 1 unit represents a decrease in the light received by a factor of 2.512.