Neutron degenerate matter is a term for the neutron-rich material within a neutron star in which nuclei have broken into their nucleons due to the high density. Other terms are used as well, such as neutron star matter and neutron matter. Neutron star material can be imaged as "one big nucleus", which gives some notion of its density, but the material has its own characteristics. The material has been described as a fluid, a liquid, or a superfluid, and generally theorized to include protons and electrons as well as neutrons. The equation of state (EoS) of the material is of interest in the study of neutron stars and the recent observation of a kilonova (GW170817) along with the potential for more such observations has increased interest. It is not known how dense the material can become yet remain stable, one clear limit being that sufficiently high density would cause formation of an event horizon/black hole, but when the EoS indicates sufficient pressure at some density short of that (a hard EoS, meaning a small density increase creates a relatively large pressure increase), this density limit is avoided. It is possible that in some neutron stars, the nucleons become so closely squeezed together that their quarks and gluons significantly affect each other, with overlapping quantum fields (overlapping wave functions). Such theoretical material has been called quark matter, but again, other terms are often used. Determinations of the densest matter in neutron stars using currently established theory is difficult, conceivably reaching a point beyond that where current theory is proven by experiment or observation; future kilonova study may reveal new physics. The transitions to these exotic types of matter under high density are phase transitions.
The term neutron matter is also sometimes used for other theoretical types of matter consisting of neutrons, e.g., a few neutrons together like an atomic nucleus but including no protons: it is clear such things exist fleetingly and the possibility of stable configurations has been explored.