Merriam-Webster Collegiate Dictionary - magnitude

 noun  Etymology: Middle English, from Anglo-French, from Latin magnitudo, from magnus  Date: 15th century  1.  a. great size or extent  b.  (1) spatial quality ; size  (2) quantity, number  2. the importance, quality, or caliber of something  3. a number representing the intrinsic or apparent brightness of a celestial body on a logarithmic scale in which an increase of one unit corresponds to a reduction in the brightness of light by a factor of 2.512  4. a numerical quantitative measure expressed usually as a multiple of a standard unit  5. the intensity of an earthquake represented by a number on an arbitrary scale a ~ six earthquake