Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H) from a Brønsted–Lowry acid in an acid–base reaction. The species formed is the conjugate base of that acid. The complementary process, when a proton is added (transferred) to a Brønsted–Lowry base, is protonation (or hydronation). The species formed is the conjugate acid of that base.
A species that can either accept or donate a proton is referred to as amphiprotic. An example is the H2O (water) molecule, which can gain a proton to form the hydronium ion, H3O, or lose a proton, leaving the hydroxide ion, OH.