Auotionization is the reaction between two molecules of the same species, forming two ionized products. In the autoionization of water, one proton from the first water molecule is donated to the second water molecules, forming $H^+$ and $OH^-$. The first $H_2O$ acts as an acid, and the second one as a base. In order to be able to undergo autoionization, a compound must be amphiprotic.
Work Step by Step
The reaction for the autoionization of water is: $$H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$$ See Section 10-5 Page 353.