1) Any member of the class of enzymes that catalyse the cleavage of the substrate and the addition of water to the resulting molecules, e.g., esterases, glycosidases (glycoside hydrolases), lipases, nucleotidases, peptidases (peptide hydrolases), and phosphatases (phosphoric monoester hydrolases).

2) Enzymes (EC class 3) cleaving substrates with addition of H2O at the point of cleavage; e.g., esterases, phosphatases, nucleases, peptidases.