any of a class of compounds containing chemically combined water. In the case of some hydrates, as washing soda, Na 2 CO 3 ⋅10H 2 O, the water is loosely held and is easily lost on heating; in others, as sulfuric acid, SO 3 ⋅H 2 O, or H 2 SO 4 , it is strongly held as water of constitution.
verb (used with object), verb (used without object), hydrated, hydrating.
to combine chemically with water.

1795–1805; hydr-1 + -ate2

hydration, noun
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World English Dictionary
hydrate (ˈhaɪdreɪt)
1.  a chemical compound containing water that is chemically combined with a substance and can usually be expelled without changing the constitution of the substance
2.  a chemical compound that can dissociate reversibly into water and another compound. For example sulphuric acid (H2SO4) dissociates into sulphur trioxide (SO3) and water (H2O)
3.  (not in technical usage) a chemical compound, such as a carbohydrate, that contains hydrogen and oxygen atoms in the ratio two to one
4.  to undergo or cause to undergo treatment or impregnation with water
[C19: from hydro- + -ate1]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1802, "compound of water and another chemical," from Fr. hydrate, coined by Fr. chemist Joseph-Louis Proust (1754-1826) from Gk. hydr-, stem of hydor "water" (see water (n.1)). The verb is first attested 1850.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

hydrate hy·drate (hī'drāt')
A solid compound containing water molecules combined in a definite ratio as an integral part of a crystal. v. hy·drat·ed, hy·drat·ing, hy·drates

  1. To rehydrate.

  2. To supply water to a person or thing in order to restore or maintain fluid balance.

hydration hy·dra·tion (hī-drā'shən)

  1. The addition of water to a chemical molecule without hydrolysis.

  2. The process of providing an adequate amount of liquid to bodily tissues.

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
hydrate   (hī'drāt')  Pronunciation Key 
Noun   A compound produced by combining a substance chemically with water. Many minerals and crystalline substances are hydrates.

  1. To combine a compound with water, especially to form a hydrate.

  2. To supply water to a person in order to restore or maintain a balance of fluids.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
They also, apparently, are fond of breaking water pipes and bathrooms in their
  quest for hydration.
Comfort care involves primarily pain management, hydration, and little else.
Your mouth will get dry, and hydration will make you feel better.
If the sole intention is to terminate feeding and hydration, there is no need
  to remove a gastric feeding tube.
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