@wikipedia

A state of Thermodynamic equilibrium between the liquid and gas state. 

The PT diagram for Pure Substance will see 

For Pure Substance describes the distribution of a chemical species between the vapor phase and a liquid phase

The end point $//<![CDATA[ \begin{array}{l}(T_c, p_c)\end{array} //]]>$ of a phase equilibrium curve above which the liquid and gas state of a pure substance do not have a distinct boundary (see Fig. 1).

It is characterized by critical temperature  $//<![CDATA[ \begin{array}{l}T_c\end{array} //]]>$ and critical pressure  $//<![CDATA[ \begin{array}{l}p_c\end{array} //]]>$ , both being a material property.

See also

Natural Science / Physics / Thermodynamics / Thermodynamic system / Thermodynamic equilibrium 

[ Pure Substance ]

[ Сritical Temperature T_c ] [ Critical Pressure (p_c) ]