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).

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

  1. variant of dia- before a vowel: diorama.


  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)

combining form

  1. variant of dia- diopter

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.


  1. Two; twice; double:dichromatic.
  2. Containing two atoms, radicals, or groups:diiodide.

  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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