What is the definition of "absolute magnitude"?

Absolute magnitude is defined as the measure of a star's intrinsic brightness. Absolute magnitude is what the apparent magnitude of a star would be if measured 10 parsecs (32.6 light years) from the Earth.

Apparent magnitude is not an accurate a measure of the brightness of a star because it is affected by the star's distance from Earth; even if a star was intrinsically very bright, it may not appear so if it is far from Earth. Absolute magnitude is a better indicator of a star's brightness because it is a better measure of the energy output of the star.