hy·dra·zin·i·um \ˌhīdrəˈzinēəm\noun (-s) Etymology: International Scientific Vocabulary hydrazine + -ium : either of two cations derived from hydrazine; especially: the univalent cation NH2NH3+ < hydrazinium chloride NH2NH3Cl or hydrazine hydrochloride NH2NH2.HCl >