The apparent brightness of an object. The lower the magnitude, the brighter the object, so magnitude 1 is brighter than magnitude 2, while magnitude -1 is brighter still. It is a logarithmic scale: each magnitude is 2.5 times as bright as the one below it, and a brightness ratio of 100 corresponds with a magnitude difference of 5.
Sirius, the brightest star, has a magnitude of -1.4. The limit of naked eye visibility is about 6. The full moon has a magnitude of about -12.7.