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From 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]