A measure of how bright an object appears when seen from Earth. It's a logarithmic scale with a difference of 5 magnitudes corresponds to a brightness change of exactly 100. A one-magnitude change is equivalent to a brightness change of 2.512. The relationship is inverse which means that the brighter an object appears, the lower its magnitude value. The apparent magnitude of stars is measured with a bolometer.
Apparent magnitude examples include:
Sun = -26.7
Full Moon = -12.9
Venus = -4.9 (max. brightness)
Sirius = -1.5 (brightest night-time star)
Faintest stars visible to naked eye (normal conditions) = +6