The minimum pressure which would suffice to liquefy a substance at its critical temperature. Above the critical pressure, increasing the temperature will not cause a fluid to vaporize to give a two-phase system.
Source:
PAC, 1993, 65, 2397. 'Nomenclature for supercritical fluid chromatography and extraction (IUPAC Recommendations 1993)' on page 2399 (https://doi.org/10.1351/pac199365112397)