Water of crystallisation, also known as hydrated water, refers to water molecules that are chemically bonded into the crystalline structure of a compound. Some synonyms for this term include water of hydration, crystalline water, or bound water. These terms reflect the fact that the water is an integral part of the crystal structure of the compound, rather than being present as a separate entity. Other related terms include anhydrate, which describes a substance that has lost its water of crystallisation, and deliquescent, which refers to substances that absorb moisture from their surroundings and dissolve in their own water of crystallisation.