The temperature and pressure at which the liquid and vapour intensive properties (density, heat capacity, etc.) become equal. It is the highest temperature (critical temperature) and pressure (critical pressure) at which both a gaseous and a liquid phase of a given compound can coexist.
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2183 (https://doi.org/10.1351/pac199062112167)
PAC, 1993, 65, 2397. 'Nomenclature for supercritical fluid chromatography and extraction (IUPAC Recommendations 1993)' on page 2399 (https://doi.org/10.1351/pac199365112397)