Onium

noun

1. In chem., a termination given to the names of compound radicals which are analogous in structure and chemical function to ammonium, NH4, as phosphonium, PH4, or tetramethylphosphonium, P (CH3)4.
2. any cation derived by the addition of a proton to the hydride of any element of the nitrogen, chalcogen or halogen families
3. any organic derivative of these compounds
4. the bound pair of a particle and its antiparticle