Hydrolysis usually means the rupture of chemical bonds by the action of water. Generally, hydrolysis is a step in the degradation of a substance. In terms of the word's derivation, hydrolysis ( /haɪˈdrɒlɨsɪs/) comes from Greek roots hydro "water" + lysis "separation".

Usually hydrolysis is a chemical process in which a molecule of water molecule adds to a substance. Sometimes this addition causes the substance to split into two parts. In such reactions, one fragment of the target molecule (or parent molecule) gains a hydrogen ion (H⁺) from the split water molecule. The other portion of the target molecule collects the hydroxyl group (OH⁻) of the split water molecule. In effect an acid and a base are formed.

A common kind of hydrolysis occurs when a salt of a weak acid or weak base (or both) is dissolved in water. Water spontaneously ionizes into hydroxyl anions and hydrogen cations. The salt, too, dissociates into its constituent anions and cations. For example, sodium acetate dissociates in water into sodium and acetate ions. Sodium ions react very little with the hydroxyl ions whereas the acetate ions combine with hydrogen ions to produce acetic acid. In this case the net result is a relative excess of hydroxyl ions, giving a basic solution.