The hydronium ion (H₃O⁺) is the ion formed when a hydrogen ion (H⁺) bonds to a water molecule; it is the species that an acid actually produces in water. Because a bare proton cannot exist alone in solution, the acidity of an aqueous solution is properly described in terms of the hydronium ion. Its concentration determines the pH of the solution.

水合氢离子(H₃O⁺)是氢离子(H⁺)与水分子结合形成的离子;它是酸在水中实际生成的物种。由于裸质子无法在溶液中单独存在,水溶液的酸性应以水合氢离子来描述。其浓度决定溶液的 pH。