magnitude

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Greatness of rank or position.
• noun Greatness in size or extent.
• noun Greatness in significance or influence.
• noun The brightness of a celestial body on a numerical scale for which brighter objects have smaller values. Differences in magnitude are based on a logarithmic scale that matches the response of the human eye to differences in brightness so that a decrease of one magnitude represents an increase in apparent brightness by a factor of 2.512.
• noun A unit on such a scale of brightness.
• noun A number assigned to a quantity so that it may be compared with other quantities.
• noun A property that can be described by a real number, such as the volume of a sphere or the length of a vector.
• noun Geology A measure of the amount of energy released by an earthquake, as indicated on the Richter scale.

from The Century Dictionary.

• noun Greatness; vastness, whether in a physical or a moral sense; grandeur.
• noun Largeness of relation or significance; importance; consequence: as, in affairs of magnitude disdain not to take counsel.
• noun Size, or the property of having size; the extended quantity of a line, surface, or solid; length, area, or volume.
• noun Any kind of continuous quantity which is comparable with extended quantity.
• noun In astronomy, the brightness of a star expressed according to the numerical system used by astronomers for that purpose.
• noun In ancient prosody, the length of a syllable, foot, colon, or meter, expressed in terms of the metrical unit (primary time, semeion, or mora): as, a foot of trisemic magnitude; a colon of icosasemic magnitude.

from the GNU version of the Collaborative International Dictionary of English.

• noun Extent of dimensions; size; -- applied to things that have length, breadth, and thickness.
• noun (Geom.) That which has one or more of the three dimensions, length, breadth, and thickness.
• noun Anything of which greater or less can be predicated, as time, weight, force, and the like.
• noun Greatness; grandeur.
• noun Greatness, in reference to influence or effect; importance.
• noun (Astron.) See magnitude of a star, below.
• noun (Opt.), (Astron.) Same as magnitude of a star, below.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun uncountable, countable The absolute or relative size, extent or importance of something.
• noun countable An order of magnitude.
• noun mathematics A number, assigned to something, such that it may be compared to others numerically
• noun mathematics Of a vector, the norm, most commonly, the two-norm.
• noun astronomy The apparent brightness of a star (on a negative, logarithmic scale); apparent magnitude
• noun seismology A measure of the energy released by an earthquake (e.g. on the Richter scale).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the property of relative size or extent (whether large or small)
• noun 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
• noun relative importance