What is a critical point?

For a pure substance the critical point marks the end of the vapour-liquid coexistence curve. Above the critical point (certain temperature and pressure), neither liquid or gas phases exist; instead, a poorly defined phase, known as a supercritical fluid, occurs. Such fluids have the gas-like characteristic of low viscosity, and the liquid-like characteristic of high density.

Measurement of Vapor-Liquid Equilibrium (VLE) data

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Which solvents are most commonly used?

Carbon dioxide is the most widely used since it has a very accessible critical temperature and pressure, is inert, and an environmentally friendly alternative to conventional solvents and readily available in high purity.