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

< Greek, combining form of hýdōr water

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a combining form representing hydrogen, in compound words, denoting especially a combination of hydrogen with some negative element or radical: hydrobromic.
Also, especially before a vowel, hydr-2.
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
hydro- or (sometimes before a vowel) hydr-
combining form
1.  indicating or denoting water, liquid, or fluid: hydrolysis; hydrodynamics
2.  indicating the presence of hydrogen in a chemical compound: hydrochloric acid
3.  indicating a hydroid: hydrozoan
[from Greek hudōr water]
hydr- or (sometimes before a vowel) hydr-
combining form
[from Greek hudōr water]

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

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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American Heritage
Science Dictionary
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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