autoprotolysis (autoionization; autoionization constant; autoprotolysis constant)

 Say it 
Transfer of a hydrogen ion between molecules of the same substance, e. g. the autoprotolysis of methanol (2 CH3OH = CH3OH2+ + CH3O-). Autoprotolysis of water into hydronium ions and hydroxide ions results in equilibrium concentrations that satisfy Kw = [H3O+][OH-], where the autoprotolysis constant Kw is equal to 1.01 × 10-14 at 25°C.