Hydronium is what you get when you put water and hydrogen ions together, forming H3O+. Hydronium is the simplest form of oxonium, which is any ion that contains the trivalent oxygen cation. Hydronium is also known as hydroxonium. As with many species in chemistry, the nomenclature isn't the same everywhere.
Where would you find hydronium? Hydronium is found in interstellar clouds and in the tails of comets. Interstellar hydronium probably forms as a result of chemical reactions following the ionization of H2 into H2+. Research is ongoing to elucidate the nature of the reactions.