Magnitude (def): In the context of a celestial body, its brightness as a measure, either apparent or absolute. Absolute magnitude is the apparent brightness of a star if it was 32.6 light years from Earth, where it’s being viewed from. It’s useful in comparing the actual brightness of celestial bodies because it involves a standard distance from observation point. This compares with apparent magnitude, which is the brightness of a star or celestial body as seen from Earth. It involves a scale system where the lower the value the greater the brightness. Objects seen by the unaided eye are those with an apparent magnitude less than about 6, depending on the observing conditions. A change in brightness on this scale is by a factor of 2.5.
For more on this, see my article on star brightness, magnitude, and luminosity, which includes graphical explanations.