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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 imagined 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 short of this, as long as the density results in enough pressure to support the weight, it remains as a neutron star. (An EoS such that a small density-increase creates a relatively large pressure-increase is termed a hard EoS.) It is possible that in some neutron stars, the nucleons become so closely squeezed together that the quarks and gluons of each significantly affect those of others, with significant overlap of their respective quantum fields. Such theoretical material has been called quark matter, but again, other terms are often used. Determining the densest matter within neutron stars is difficult and speculative; density conceivably reaches a point beyond that in which theory is proven by experiment or observation. Future kilonova study may reveal information about it. The transitions producing such exotic types of matter under high density are examples of 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.