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mega-

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  1. 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
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Also especially before a vowel, meg-.

Origin of mega-

combining form representing Greek mégas large, great
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for mega

mega

adjective
  1. slang extremely good, great, or successful
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Word Origin

C20: probably independent use of mega-

mega-

combining form
  1. denoting 10 6megawatt Symbol: M
  2. (in computer technology) denoting 2 20 (1 048 576)megabyte
  3. large or greatmegalith
  4. informal great in importance or amountmegastar
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Word Origin

from Greek megas huge, powerful
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for mega

mega-

before vowels meg-, word-forming element often meaning "large, great," but in precise scientific language "one million" (megaton, megawatt, etc.), from Greek megas "great, large, vast, big, high, tall; mighty, important" (fem. megale), from PIE *meg- "great" (cf. Latin magnus, Old English micel; see mickle). Mega began to be used alone as an adjective by 1982.

High-speed computer stores 2.5 megabits [headline in "Electronics" magazine, Oct. 1, 1957]
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Online Etymology Dictionary, © 2010 Douglas Harper

mega in Medicine

mega-

pref.
  1. Large:megacephaly.
  2. One million (106):megahertz.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

mega in Science

mega-

  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.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.