Meaning of water of hydration

wa'ter of hydra'tion

Pronunciation: [key]
— Chem. Chem.
  1. the portion of a hydrate that is represented as, or can be expelled as, water: now usually regarded as being in true molecular combination with the other atoms of the compound, and not existing in the compound as water.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.