What is a critical point in a phase diagram?

A critical point in a phase diagram is defined as the specific temperature and pressure at which a substance's gas phase can no longer be distinguished from its liquid phase. At this point, called the critical point, the properties of the liquid and gas become indistinguishable, and the substance exists as a supercritical fluid.

As the pressure and temperature exceed this critical point, it is impossible to convert the gas back to a liquid, regardless of how much pressure is applied. This phenomenon is essential in understanding phase transitions and behavior of substances under extreme conditions. In contrast, the other options describe different phase transition behaviors that do not capture the unique characteristics of the critical point.