| Hydronium, a weak acid, has a chemical formula of H3O+. A more descriptive definition would say that hydronium is water with an added H+ ion. It almost always exists in an aqueous state.
Hydronium is chemically interesting because it is the strongest that can exist in water without neutralization. All other acids will react with water acting as the base. The reason hydronium can stave off this autoreaction is that it's actually a conjugate acid of water. Since water is amphoteric, it can act as either an acid or base depending on circumstances, and the instability of hydronium means that in a conjugate relationship water will actually be a strong base to hydronium's weak acid.
Hydronium has a practical medical application as a disinfectant, similar to hydrogen peroxide. Beyond this there aren't many uses for hydronium outside chemistry experiments with acids and bases. It does not occur commonly in nature. | 
