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co-

  1. variant of com- before a vowel, h, and gn: coadjutor; cohabit; cognate. The prefix co- now productively forms new words from bases beginning with any sound (co-conspirator; co-manage; coseismic), sometimes with the derived sense “auxiliary, subsidiary” (coenzyme; copilot), and, in mathematics and astronomy, with the sense “complement” (codeclination).
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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for co-

co-

prefix
  1. together; joint or jointly; mutual or mutuallycoproduction
  2. indicating partnership or equalitycofounder; copilot
  3. to the same or a similar degreecoextend
  4. (in mathematics and astronomy) of the complement of an anglecosecant; codeclination
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Word Origin for co-

from Latin, reduced form of com-
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 co-

in Latin, the form of com- in compounds with stems beginning in vowels and h- and gn- (see com-). Taken in English from 17c. as a living prefix meaning "together, mutually, in common," and used promiscuously with native words and Latin-derived words not beginning with vowels, sometimes even with words already having it (e.g. co-conspiritor).

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Online Etymology Dictionary, © 2010 Douglas Harper

co- in Medicine

co-

pref.
  1. Together; joint; jointly; mutually:coaptation.
  2. Subordinate or auxiliary:coenzyme.
  3. To the same extent or degree:codominant.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.