https://doi.org/10.1351/goldbook.H02876
Addition of water or of the elements of water (i.e. H and OH) to a molecular entity. For example, hydration of ethene: 
The term is also used in a more restricted sense for the process: 
Note:
Compare the use of the term in inorganic/physical chemistry to describe the state of the ions of an electrolyte in aqueous solution.
Compare the use of the term in inorganic/physical chemistry to describe the state of the ions of an electrolyte in aqueous solution.