The critical point of a substance is the highest temperature and pressure at which it can exist as a distinct liquid and gas phase. Beyond this point, the substance exists as a supercritical fluid, where the liquid and gas phases merge into a single phase with properties of both. The critical point is characterized by the critical temperature (the temperature above which a gas cannot be liquefied, no matter how much pressure is applied) and the critical pressure (the pressure required to liquefy a gas at its critical temperature).

For example, the critical point for water is approximately 374°C (705°F) and 22.1 MPa (about 218 times atmospheric pressure). At this point, water exhibits unique properties different from those of ordinary liquids and gases.