Only a few stars are large enough and close enough that their radii can be measured directly. The absolute brightness of a star is equivalent to its luminosity. The apparent brightness of a star is the rate at which energy from the star reaches unit area of a detector. Apparent brightness falls off as the inverse square of the distance. Optical astronomers use the magnitude scale to express and compare stellar brightnesses.
Apparent magnitude is a measure of apparent brightness.
The absolute magnitude of a star is the apparent magnitude it would have if placed at a standard distance of 10 pc from the viewer. It is a measure of the star's luminosity.