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demi-

  1. a combining form appearing in loanwords from French meaning “half” (demilune), “lesser” (demitasse), or sometimes used with a pejorative sense (demimonde); on this model, also prefixed to words of English origin (demigod).
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Origin of demi-

< French, combining form representing demi (adj.; also noun and adv.) < Vulgar Latin *dīmedius, for Latin dīmidius half, equivalent to dī- di-2 + medius middle
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for demi-

demi-

prefix
  1. halfdemirelief Compare hemi-, semi- (def. 1)
  2. of less than full size, status, or rankdemigod
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Word Origin for demi-

via French from Medieval Latin dīmedius, from Latin dīmīdius half, from dis- apart + medius middle
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 demi-

word-forming element meaning "half, half-sized, partial," early 15c., from Old French demi "half" (12c.), from Late Latin dimedius, from Latin dimidius "half, one-half," which contains the elements dis- "apart" (see dis-) + medius "middle" (see medial).

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Online Etymology Dictionary, © 2010 Douglas Harper
demi- in Medicine

demi-

pref.
  1. Half:demilune.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.