A transparent, superfluid and super-dense state of matter, in between electron degenerate matter and quark degenerate matter. It is composed entirely of neutrons. Neutronium occurs naturally in neutron stars, where the gravitational pressure is sufficient to force such a state and is unstable at lesser pressures.
An exotic form of neutronium exists in certain neutron stars capable of forming complex structures; this material is the basis for the xenosophont race known as Hildemar's Knots. This exotic material cannot exist outside of the extreme pressures of a neutron star, so is not useful for construction projects outside of this environment.