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a combining form meaning “outside,” “outer,” “external,” used in the formation of compound words:
Also, ex-.
Origin of exo-
< Greek, combining form of éxō outside Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for exo-


combining form
external, outside, or beyond: exobiology, exothermal
Word Origin
from Greek exō outside
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 exo-

word-forming element used in scientific words (e.g. exoskeleton), from Greek exo "outside," related to ex "out of" (see ex-).

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

exo- pref.
Outside; external: exoskeleton.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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exo- in Science
A prefix that means "outside" or "external," as in exoskeleton.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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