The Widom line has been suggested to separate liquid-like behaviour and gas-like behaviour in supercritical fluids, where traditional phase boundaries between liquid and gas no longer exist. Specifically, the supercritical fluid has gas-like behavior on the low-pressure side of the Widom line and liquid-like behavior on the high-pressure side of the Widom line. This separation is a continuous crossover in some of the properties of the fluid and has been observed in laboratory experiments, for example on fluid methane. The concept of Widom line provides a useful framework for characterizing and predicting the properties of fluids, which are important for scientific research as well as various industrial processes. Such a concept is indeed relevant to the physical properties of any single-component fluid at sufficiently high pressures and temperatures, and its study is an active and ongoing research area.