Noun
(informal, nuclear physics, science fiction) Synonym of neutronium (“material composed of neutrons”)
Coordinate terms: neutron, free neutron, tetraneutron, neutron star, neutron star matter, neutron-degenerate matter, neutron matter, neutron degeneracy, n
(informal, nuclear chemistry) Synonym of muonium
Coordinate terms: protium, deuterium, tritium, hydrium, hydrogen-0, hydrogen-1, hydrogen-2, hydrogen-3, hydrogen, muon, anti-muon, muononium, exotic atom, atom, onium, meson
(science fiction) A fictional chemical element with element number/atomic number/proton number zero (0). (This frequently has magical unreal impossible characteristics.)
Used other than figuratively or idiomatically: see element, zero.
Source: en.wiktionary.org