In the context of the temperature-pressure phase diagram of a substance and of the supercritical fluid state in particular, the Widom line is a line emanating from the critical point which somehow extends the liquid-vapor coexistence curve above the critical point. It corresponds to the maxima or minima in certain physical properties of the supercritical fluid, such as the speed of sound, isothermal compressibility, isochoric and isobaric heat capacities. A common criterion for locating the Widom line is indeed the maximum in the isobaric heat capacity.