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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).
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-


variant of dis-1. before b, d, l, m, n, r, s, v, and sometimes g and j: digest; divide.


variant of dia- before a vowel:
diorama. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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British Dictionary definitions for di-


twice; two; double: dicotyledon
  1. containing two specified atoms or groups of atoms: dimethyl ether, carbon dioxide
  2. a nontechnical equivalent of bi-1 (sense 5c)
Word Origin
via Latin from Greek, from dis twice, double, related to duo two. Compare bi-1


combining form
variant of dia- diopter
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
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Word Origin and History for di-

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

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

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

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

di- pref.

  1. Two; twice; double: dichromatic.

  2. Containing two atoms, radicals, or groups: diiodide.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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di- in Science
A prefix that means "two," "twice," or "double." It is used commonly in chemistry, as in dioxide, a compound having two oxygen atoms.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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