The Collaborative International Dictionary of English v.0.48:

Magnitude \Mag"ni*tude\, n. [L. magnitudo, from magnus great.
   See Master, and cf. Maxim.]
   1. Extent of dimensions; size; -- applied to things that have
      length, breadth, and thickness.
      [1913 Webster]

            Conceive those particles of bodies to be so disposed
            amongst themselves, that the intervals of empty
            spaces between them may be equal in magnitude to
            them all.                             --Sir I.
                                                  Newton.
      [1913 Webster]

   2. (Geom.) That which has one or more of the three
      dimensions, length, breadth, and thickness.
      [1913 Webster]

   3. Anything of which greater or less can be predicated, as
      time, weight, force, and the like.
      [1913 Webster]

   4. Greatness; grandeur. "With plain, heroic magnitude of
      mind." --Milton.
      [1913 Webster]

   5. Greatness, in reference to influence or effect;
      importance; as, an affair of magnitude.
      [1913 Webster]

            The magnitude of his designs.         --Bp. Horsley.
      [1913 Webster]

   6. (Astron.) See magnitude of a star, below.
      [PJC]

   Apparent magnitude

   1. (Opt.), the angular breadth of an object viewed as
      measured by the angle which it subtends at the eye of the
      observer; -- called also apparent diameter.

   2. (Astron.) Same as magnitude of a star, below.

   Magnitude of a star (Astron.), the rank of a star with
      respect to brightness. About twenty very bright stars are
      said to be of first magnitude, the stars of the sixth
      magnitude being just visible to the naked eye; called also
      visual magnitude, apparent magnitude, and simply
      magnitude. Stars observable only in the telescope are
      classified down to below the twelfth magnitude. The
      difference in actual brightness between magnitudes is now
      specified as a factor of 2.512, i.e. the difference in
      brightness is 100 for stars differing by five magnitudes.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

magnitude
    n 1: the property of relative size or extent (whether large or
         small); "they tried to predict the magnitude of the
         explosion"; "about the magnitude of a small pea"
    2: a number assigned to the ratio of two quantities; two
       quantities are of the same order of magnitude if one is less
       than 10 times as large as the other; the number of magnitudes
       that the quantities differ is specified to within a power of
       10 [syn: order of magnitude, magnitude]
    3: relative importance; "a problem of the first magnitude"

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "magnitude":
   Beehive, Cepheid variable, Hertzsprung-Russell diagram, Hyades,
   Messier catalog, NGC, Pleiades, Seven Sisters, absolute magnitude,
   amount, ampleness, amplitude, area, bigness, binary star,
   black hole, body, boundlessness, breadth, bulk, caliber,
   consequence, coverage, depth, diameter, dimension, dimensions,
   double star, dwarf star, enormity, enormousness, expanse,
   expansion, extension, extent, fixed star, force, formidableness,
   fullness, gauge, giant star, gigantism, girth, globular cluster,
   grandeur, grandness, gravity star, great scope, greatness, height,
   hugeness, immensity, import, importance, infinity, intensity,
   largeness, length, main sequence star, mass, mass-luminosity law,
   matter, measure, measurement, might, mightiness, moment,
   momentousness, muchness, neighborhood, neutron star, note, nova,
   numbers, open cluster, order, pith, plenitude, populations, power,
   prodigiousness, proportion, proportions, pulsar, quantity, quantum,
   quasar, quasi-stellar radio source, radio star, radius, range,
   reach, red giant star, relative magnitude, scale, scope,
   significance, signification, size, sky atlas,
   spectrum-luminosity diagram, spread, star, star catalog,
   star chart, star cloud, star cluster, stellar magnitude, strength,
   stupendousness, substance, sum, supernova, tour de force,
   tremendousness, tune, variable star, vastness, vicinity, volume,
   weight, weightiness, white dwarf star, whole, width



The Devil's Dictionary (1881-1906):

MAGNITUDE, n.  Size.  Magnitude being purely relative, nothing is
large and nothing small.  If everything in the universe were increased
in bulk one thousand diameters nothing would be any larger than it was
before, but if one thing remain unchanged all the others would be
larger than they had been.  To an understanding familiar with the
relativity of magnitude and distance the spaces and masses of the
astronomer would be no more impressive than those of the microscopist.
For anything we know to the contrary, the visible universe may be a
small part of an atom, with its component ions, floating in the life-
fluid (luminiferous ether) of some animal.  Possibly the wee creatures
peopling the corpuscles of our own blood are overcome with the proper
emotion when contemplating the unthinkable distance from one of these
to another.