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-.

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

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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.
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World English Dictionary
1.  twice; two; double: dicotyledon
2.  a.  containing two specified atoms or groups of atoms: dimethyl ether; carbon dioxide
 b.  a nontechnical equivalent of bi-
[via Latin from Greek, from dis twice, double, related to duo two. Compare bi-1]

combining form
variant of dia- : diopter

dia- or di-
1.  through, throughout, or during: diachronic
2.  across: diactinic
3.  apart: diacritic
4.  (in botany) at right angles: diatropism
5.  in opposite or different directions: diamagnetism
[from Greek dia through, between, across, by]
di- or di-
[from Greek dia through, between, across, by]

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Medical Dictionary

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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American Heritage
Science Dictionary
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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