Wiktionary
n. (context physics English) the supposed material composed entirely of neutrons that composes a neutron star
Wikipedia
Neutronium (sometimes shortened to neutrium) is a proposed name for a substance composed purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 (before the discovery of the neutron) for the conjectured "element of atomic number zero" that he placed at the head of the periodic table. However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been also used legitimately to refer to extremely dense substances resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars; hereinafter "degenerate neutronium" will refer to this. Science fiction and popular literature frequently use the term "neutronium" to refer to a highly dense phase of matter composed primarily of neutrons.
Usage examples of "neutronium".
At her urging he described it as best he could: the colors, the progressive flattening of core suns into an accretion disk, the swelling of the Ring of Fire, the final drastically flattened plane of neutronium flecked with smaller black holes.
He'd explain the disintegration field, which collapses the electrons of hydrogen so that it rises in atomic weight to helium, and the helium to lithium, while the oxygen of the water is split literally into neutronium and pure force.
The vast majority of superfluids are either cryogenic, as with liquid helium, or supercondensed, as with neutronium.