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a combining form meaning “water,” used in the formation of compound words:
hydroplane; hydrogen.
Also, especially before a vowel, hydr-1 .
Origin of hydro-1
< Greek, combining form of hýdōr water


a combining form representing hydrogen, in compound words, denoting especially a combination of hydrogen with some negative element or radical:
Also, especially before a vowel, hydr-2 . Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for hydro-


combining form
indicating or denoting water, liquid, or fluid: hydrolysis, hydrodynamics
indicating the presence of hydrogen in a chemical compound: hydrochloric acid
indicating a hydroid: hydrozoan
Word Origin
from Greek hudōr water
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 hydro-

before vowels hydr-, word-forming element meaning "water," from Greek hydro-, comb. form of hydor "water" (see water (n.1)). Also sometimes a comb. form of hydrogen.

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

hydro- or hydr-

  1. Water; liquid: hydrocephalus.

  2. Hydrogen: hydrochloride.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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hydro- in Science
A prefix that means: "water" (as in hydroelectric) or "hydrogen," (as in hydrochloride).
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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