1. A solid compound containing water molecules combined in a definite ratio as an integral part of the crystal.
2. To combine chemically with water.
3. To supply water to a person or thing in order to restore or to maintain fluid balance.

A cage-like arrangement of water molecules stabilized by a central guest molecule.

Such hydrates exist at higher pressures in glaciers where air bubbles trapped in the process of firn formation (old snow that has become granular and compacted or dense) near the surface and appear to dissolve in the ice.