The description of the word you requested from the astronomical dictionary is given below.
In general, "magnitude" means about the same as "importance". In science, a magnitude is usually a logarithmic measure, such as for the strength of an earthquake or the brightness of a star.
In astronomy, the magnitude or apparent magnitude is a measure for how bright something appears to be (at the distance that it has). The brightest stars in the sky have a magnitude of about 0 or 1. The dimmest stars that someone with good eyesight can just make out under good conditions (far from city lights and other lights) without optical aids (such as field glasses or a telescope) have a magnitude of about 6 (are of about the sixth magnitude). The greater (more positive) the magnitude, the less is the brightness.
The scale of magnitudes is a logarithmic scale. A difference of 1 magnitude corresponds to a ratio of about 2.5, and a difference of 5 magnitudes corresponds to exactly a ratio of 100: A star of magnitude 0 appears 100 times brighter than a star of magnitude 5. Stars and other things can also have a negative magnitude. The Sun has a magnitude of about −26.7 and the average full moon has a magnitude of about −12.6. Venus is of about magnitude −4 at its brightest, and the brightest star in the sky, Sirius, has magnitude −1.5. The dimmest things seen so far (with large telescopes and long exposure times) have a magnitude of about +31.