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Wiktionary
onium

n. 1 (context chemistry English) any cation derived by the addition of a proton to the hydride of any element of the nitrogen, chalcogen or halogen families 2 (context chemistry English) any organic derivative of these compounds

Wikipedia
Onium

An onium (plural: onia) is a bound state of a particle and its antiparticle. They are usually named by adding the suffix -onium to the name of the constituting particle except for muonium which, despite its name, not a bound muon–antimuon onium, but an electron–antimuon bound state, and whose name was assigned by IUPAC. A muon–antimuon onium would be named true muonium or muononium.

Usage examples of "onium".

This is a situation which agrees with no other city in all Egypt, except that which was the Onium of the later Jews.