water of crystallization

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Water of hydration that is necessary for the maintenance of a particular crystalline structure but capable of being removed by sufficient heat.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry The water present in the crystals of the salts of certain metals; it is weakly bound by electrostatic forces and may normally be removed by heating.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the water present in hydrated compounds