A predicted state of nuclear matter containing deconfined quarks and gluons. According to the theory of strong interactions, called quantum chromodynamics, hadrons such as mesons and nucleons (the generic name for protons and neutrons) are bound states of more fundamental objects called quarks. The quarks are confined within the individual hadrons by the exchange of particles called gluons. However, calculations indicate that, at sufficiently high temperatures or densities, hadronic matter should evolve into a new phase of matter containing deconfined quarks and gluons, called a quark-gluon plasma when hot, or quark matter when cold and dense. Such a state of matter is thought to have existed briefly in the very early universe until about 10 microseconds after the big bang, and its cold form might exist today inside the dense cores of neutron stars. One important prediction is that the mechanism, called spontaneous chiral symmetry breaking, that is thought to be responsible for 98% of the mass of the nucleon under normal conditions is not operative in the quark-gluon plasma.