• synonyms


  1. a prefix occurring in loanwords from Greek, where it meant “two,” “twice,” “double” (diphthong); on this model, freely used in the formation of compound words (dicotyledon; dipolar) and in chemical terms (diatomic; disulfide).
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Also dis-2.
Compare mono-.

Origin of di-1

Middle EnglishLatin < Greek, combining form representing dís twice, double, akin to dýo two. See bi-1, twi-


  1. variant of dis-1 before b, d, l, m, n, r, s, v, and sometimes g and j: digest; divide.
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  1. variant of dia- before a vowel: diorama.
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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for di-


  1. twice; two; doubledicotyledon
    1. containing two specified atoms or groups of atomsdimethyl ether; carbon dioxide
    2. a nontechnical equivalent of bi- 1 (def. 5c)
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Word Origin

via Latin from Greek, from dis twice, double, related to duo two. Compare bi- 1


combining form
  1. variant of dia- diopter
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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 di-


word-forming element meaning "two, double, twice," from Greek di-, from dis "twice," related to duo (see two).

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word-forming element meaning "apart, asunder," form of dis- before certain voiced consonants.

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word-forming element meaning "through; thoroughly," form of dia- before vowels.

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Online Etymology Dictionary, © 2010 Douglas Harper

di- in Medicine


  1. Two; twice; double:dichromatic.
  2. Containing two atoms, radicals, or groups:diiodide.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

di- in Science


  1. A prefix that means “two,” “twice,” or “double.” It is used commonly in chemistry, as in dioxide, a compound having two oxygen atoms.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.