noun

1. greatness; specif.,
   1. of size
   2. of extent
   3. of importance or influence
   4. Obsolete of character
2. 1. size or measurable quantity the magnitude of a velocity
   2. loudness (of sound)
   3. importance or influence
3. Astron. a number representing the apparent brightness of a celestial body: it is part of an unlimited arbitrary scale that ranges from the brightest object, the sun, at -26.72 to the faintest visible object at c. 26: only 22 stars are brighter than 1.5 (first magnitude), while stars c. 6 (sixth magnitude) are barely visible to the naked eye: each increase of one magnitude equals 2.512 times as much brightness (a magnitude increase of 5 is 100 times brighter)
4. Geol. a measure of the amount of energy released by an earthquake
5. Math. a number given to a quantity for purposes of comparison with other quantities of the same class

Etymology: L magnitudo < magnus, great: see magni-