An Etymological Dictionary of Astronomy and Astrophysics

A → water, → molecule with an additional hydrogen ion (H₃O⁺). Also called hydronium ion. Hydronium is an abundant molecular ion in the interstellar diffuse and dense molecular clouds (→ Sagittarius B2, → Orion molecular cloud OMC-1) as well as the plasma tails of → comets (→ Halley, → Hale-Bopp).

See also: From hydr-, → hydro- + -onium a suffix used in the names of complex cations, extrcated from ammonium “ionized ammonia” (NH₄⁺).