nounScience Fiction Astronomy
Very dense matter consisting of free neutrons, thought to exist in the interior of neutron stars where electrons and protons combine with each other as a result of extreme pressure.
In Science Fiction frequently conceived of as a solid. Astrophysical research suggests that the condensed neutron material present in neutron stars is predominantly in a superfluid state.
1930s. From neutron + -ium, after German Neutronium.