Neutronium is the term for an extremely dense phase of matter which naturally occurs in the intense pressure found in the core of neutron stars. When a massive star creates an iron core whose mass exceeds the maximum mass of a white dwarf (approximately 3×10³⁰ kg), it will collapse the core with enough pressure to compress the electrons of the iron atoms into their nuclei where they combine with protons to form neutrons.

In this process, huge amounts of neutrinos are ejected, leaving a material behind with a density of approximately 10¹⁴-10¹⁵ grams per cubic centimeter. A teaspoon full of this matter would have a mass of 100 million metric tons. Beyond this, there is a limit at which a neutron star can no longer support itself and would collapse all the way into a black hole.

Because naturally occurring neutronium is bound together by gravity rather than the Strong Nuclear Force, it is immune to the nuclear disruptive effect of nadions from phasers and disruptors. Neutronium was the main component of the hull of the Planet-Killer encountered by the original USS Enterprise, and was a common component of Iconian buildings.

A form of artificial neutronium has also been produced and is sometimes used in the construction of starships or other space facilities in the Galactic Empire; it has also been alloyed with carbon to produce the structure of the Dyson's Sphere encountered by the Enterprise-D. Federation science has yet to discover a way to manipulate artificial neutronium for practical military use.

Artificially constructed neutronium is simply a matrix of neutrons held together by some basic subatomic particle or force other than gravity. Normally it does not have or require ultra-concentrated gravity fields, simply because the neutrons in these artificial substances are not packed as closely together as in naturally occurring neutronium. But at the same time, this neutron matrix, held together by whichever means, provides many of the same protections that natural neutronium would provide, but in less potency.