State at which two phases of a substance first become indistinguishable.
- The temperature or pressure at which a change in crystal structure, phase or physical properties occurs. Same as transformation temperature.
- In the equilibrium diagram, that specific value of composition, temperature and pressure, or combinations thereof, at which the phases of a heterogeneous system are in equilibrium.
- At pressures higher than 217.6atm and temperatures above 374°C, the meniscus between steam and liquid water will vanish; the two phases become indistinguishable and are referred to as a supercritical fluid.