ox·o·ni·um \äkˈsōnēəm\noun (-s) Etymology: New Latin, from ox- (I) + -onium : the univalent cation H3O+ derived from oxygen and known especially in the form of organic derivatives : the monohydrated hydrogen ion < diethyl-oxoium chloride [(C2H5)2HO]+Cl- > — called also hydronium