An Etymological Dictionary of Astronomy and Astrophysics
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1) Thermodynamics: Describing a condition in which a substance has a temperature or pressure above its critical value of temperature or pressure.
2) Nuclear physics: Of or relating to an arrangement of → fissile material where more neutrons are being produced than are wasted and escape.

→ super-; → critical.

A fluid that is at a temperature and pressure above its thermodynamic critical point. In these conditions the substance acquires unique characteristics of density and mobility. Supercritical fluids exist deep inside some planets; for example, there is supercritical water deep inside the Earth.

→ supercritical; → fluid.