A supercritical fluid is a substance under pressure above its critical temperature. Under these conditions the distinction between gases and liquids does not apply and the substance can only be described as fluid.
The photograph on the left shows a substance below its critical temperature existing as a liquid with the gas or vapour above it. As the temperature is raised, the liquid density falls due to expansion and the gas density rises as more of the substance evaporates. The densities approach each other and the meniscus between the two phases becomes less distinct, as shown in the middle photograph. Eventually, at the critical point, the densities become identical, as shown on the right. Other properties also become identical and the distinction between liquid and gas disappears, like the meniscus. The substance is now a supercritical fluid.
Supercritical fluids have properties intermediate between those of gases and liquids, controlled by the pressure, and these may be optimum for some processes. Carbon dioxide is available as a convenient supercritical fluid substance, which offers environmental advantages, as it can replace organic solvents, reducing pollution and avoiding solvent residues in products. The carbon dioxide used is a by-product of other processes and does not add to global warming.