[mag-ni-tood, -tyood]
size; extent; dimensions: to determine the magnitude of an angle.
great importance or consequence: affairs of magnitude.
greatness of size or amount.
moral greatness: magnitude of mind.
Also called visual magnitude, apparent magnitude. the brightness of a star or other celestial body as viewed by the unaided eye and expressed by a mathematical ratio of 2.512: a star of the first magnitude is approximately 2½ times as bright as one of the second magnitude and 100 times brighter than one of the sixth magnitude. Only stars of the sixth magnitude or brighter can be seen with the unaided eye.
Mathematics. a number characteristic of a quantity and forming a basis for comparison with similar quantities, as length.
of the first magnitude, of utmost or major importance: an artist of the first magnitude.

1350–1400; Middle English < Latin magnitūdō. See magni-, -tude

magnitudinous [mag-ni-tood-n-uhs, -tyood-] , adjective
overmagnitude, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
magnitude (ˈmæɡnɪˌtjuːd)
1.  relative importance or significance: a problem of the first magnitude
2.  relative size or extent: the magnitude of the explosion
3.  maths a number assigned to a quantity, such as weight, and used as a basis of comparison for the measurement of similar quantities
4.  astronomy absolute magnitude See also visual magnitude Also called: apparent magnitude the apparent brightness of a celestial body expressed on a numerical scale on which bright stars have a low value. Values are measured by eye (visual magnitude) or more accurately by photometric or photographic methods, and range from --26.7 (the sun), through 1.5 (Sirius), down to about +30. Each integral value represents a brightness 2.512 times greater than the next highest integral value
5.  geology See Richter scale Also called: earthquake magnitude a measure of the size of an earthquake based on the quantity of energy released: specified on the Richter scale
[C14: from Latin magnitūdō size, from magnus great]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

late 14c., from L. magnitudo "greatness, bulk, size," from magnus "great" (see magnate) + -tudo, suffix forming abstract nouns from adjectives and participles.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
magnitude   (māg'nĭ-td')  Pronunciation Key 
  1. The degree of brightness of a star or other celestial body, measured on a logarithmic scale in which lower numbers mean greater brightness, such that a decrease of one unit represents an increase in brightness by a factor of 2.512. An object that is 5 units less than another object on the magnitude scale is 100 times more luminous. Because of refinements in measurement after the zero point was assigned, very bright objects have negative magnitudes. ◇ The brightness of a celestial body as seen from Earth is called its apparent magnitude. (When unspecified, an object's magnitude is normally assumed to be its apparent magnitude.) The dimmest stars visible to the unaided eye have apparent magnitude 6, while the brightest star in the night sky, Sirius, has apparent magnitude -1.4. The full Moon and the Sun have apparent magnitudes of -12.7 and -26.8 respectively. ◇ The brightness of a celestial body computed as if viewed from a distance of 10 parsecs (32.6 light-years) is called its absolute magnitude. Absolute magnitude measures the intrinsic brightness of a celestial object rather than how bright it appears on Earth, using the same logarithmic scale as for apparent magnitude. Sirius has an absolute magnitude of 1.5, considerably dimmer than Rigel which, though its apparent magnitude is 0.12, has an absolute magnitude of -8.1. Stars that appear dim in the night sky but have bright absolute magnitudes are much farther from Earth than stars that shine brightly at night but have relatively dim absolute magnitudes. The Sun, a star of only medium brightness, has an absolute magnitude of 4.8. ◇ The degree of total radiation emitted by a celestial body, including all infrared and ultraviolet radiation in addition to visible light, is called its bolometric magnitude. Bolometric magnitude is generally measured by applying a standard correction to an object's absolute magnitude.

  2. A measure of the total amount of energy released by an earthquake, as indicated on the Richter scale. See more at Richter scale.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Although the magnitude of these dunes and the depth of the water is certainly
  striking, what surprised me more was the grain size.
Most people are only comfortable with orders of magnitude close to the size of
  the human body.
No magnitude correction is needed for these values.
No other educational and cultural institution has visitation of this magnitude.
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