Prior to a general discussion on thermophysical properties of fluids at critical and supercritical pressures it is important to define special terms and expressions used at these conditions. For better understanding of these terms and expressions a thermodynamic diagram is shown in Figure 1.

Figure 1 Pressure-temperature diagram for water

A. Definitions of selected terms and expressions related to critical and supercritical regions¹

Compressed fluid is a fluid at a pressure above the critical pressure but at a temperature below the critical temperature.

Critical point (also called a critical state) is the point where the distinction between the liquid and gas (or vapor) phases disappears, i.e., both phases have the same temperature, pressure and volume. The critical point is characterized by the phase state parameters T_cr, p_cr and V_cr, which have unique values for each pure substance.