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-

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

Definition for di- (2 of 3)


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

Definition for di- (3 of 3)


variant of dia- before a vowel: diorama.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

British Dictionary definitions for di- (1 of 2)



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

Word Origin for di-

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

British Dictionary definitions for di- (2 of 2)


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

Medical definitions for di-



Two; twice; double:dichromatic.
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.

Scientific definitions for di-


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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.