A new state of nuclear matter in which the recognizable components are not the familiar neutrons and protons, but are the quarks and the gluons through which they interact. The development of Quantum Chromodynamics (QCD) as a fundamental theory of strong interactions provides firm predictions that at extremely high (but apparently achievable) values of temperature and density, ordinary nuclear matter can be transformed into a plasma of quarks and gluons. It is believed that the early universe consisted of such a plasma before a phase transition occurred approximately 10 microseconds after the Big Bang. Physicists in the field of Relativistic Heavy Ions hope to create and study this new state of matter by colliding heavy nuclei at extremely high energies.

References
J.W. Harris and B. Müller, The Search for the Quark-Gluon Plasma, ARNPS 46 (1996)