The critical temperature is that temperature above which unique liquid and gas phases do not exist. As you approach the critical temperature, the properties of the gas and liquid phases become the same, so above the critical temperature there is only one phase. The critical pressure refers to the vapor pressure at the critical temperature. The critical molar volume is the volume of one mole of material at the critical temperature and pressure.
Critical properties vary from material to material, just as is the case for the melting point and boiling point. Critical properties for many pure substances are readily available in the literature. Obtaining critical properties for mixtures is somewhat more problematic.
For the case of pure substances, there is an inflexion point in the critical isotherm on a PV diagram. This means that at the critical point:
Sometimes a set of reduced properties are defined in terms of the critical properties, ie.:
Two immiscible liquids, such as oil and water, will also have a critical temperature and pressure at which the two phases will become consolute.
See also Critical exponents.