The critical point of a substance, also known as the critical state, is the temperature and pressure at which the gas and liquid phase of the substance have the same density. At that point, the gas and liquid phases of the substance are indistinguishable from one another.
If any one parameter, whether it is temperature or pressure, is changed once a substance has reached its critical point, the entire substance changes phase to be either a liquid or gas, but never both. Substances which are heated and pressed above their critical point such that they remain in a state of being indistinguishable as either liquid or gas are called supercritical fluids. Supercritical fluids are used as solvents in reaction chemistry and for dividing and extracting solids from solution.