Auto ionization of water is also called self-ionization of water where in the water molecules tend to ionize itself in an aqueous solution. In this phenomenon, one H2O molecule loses its proton (H+) and another H2O molecule accepts the proton which was earlier released. The equation for the auto ionization of water can be written as:
H2O + H2O <----> H3O+ + OH-
It is a reversible process (indicated by the double arrow sign). In general this reaction is called autoprotolysis which is a characteristic of amphoteric substances like water.