Hydrate is a term which means different things in inorganic chemistry and organic chemistry.

In organic chemistry, a hydrate is a compound formed by the addition of water to a host molecule. Thus ethanol could be considered to be the hydrate of ethylene. These substances do not contain water as such, but have their constituents (hydrogen, oxygen, hydroxyl) so arranged that water may be eliminated. Hence, hydrates are derivatives of, or compounds with, hydroxyl.

In inorganic chemistry, hydrates contain water molecules that are either bound to a metal center or crystallize with the metal complex. Such hydrates are also said to contain "water of crystallization". If the water is heavy water, where the hydrogen consists of the isotope deuterium, then the term deuterate may be used in place of hydrate.