variant of megalo- (megalith ); also the initial element in units of measure that are equal to one million of the units denoted by the base word (megahertz ). Symbol: M
Also, especially before a vowel, meg-.

combining form representing Greek mégas large, great Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
mega (ˈmɛɡə)
slang extremely good, great, or successful
[C20: probably independent use of mega-]

combining form
1.  M denoting 106: megawatt
2.  (in computer technology) denoting 220 (1 048 576): megabyte
3.  large or great: megalith
4.  informal great in importance or amount: megastar
[from Greek megas huge, powerful]

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

prefix, often meaning "large, great," but in precise scientific language "one million" (megaton, megawatt, etc.), from Gk. megas "great, large, mighty" (fem. megale), from PIE *meg- "great" (cf. L. magnus, Goth. mikils, O.E. micel; see mickle). Mega began to be used alone as an adj. 1982.
"High-speed computer stores 2.5 megabits" [headline in "Electronics" magazine, Oct. 1, 1957]
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

mega- pref.

  1. Large: megacephaly.

  2. One million (106): megahertz.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
  1. A prefix that means:

  2. Large, as in megadose, a large dose.

  3. One million, as in megahertz, one million hertz.

  4. 220 (that is, 1,048,576), which is the power of 2 closest to a million, as in megabyte.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Have you considered using insulin and liquid mega vitamins.
Bring back the small farms, that would end unemployment, feed the nation and
  stop the pollution from mega factory farms.
My kids and all of their friends got through middle school without the issues
  that plagued many in my big mega middle school.
The mega map occupies a large wall, or can be used on the floor.
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