what is the critical point on a phase diagram
It is therefore called the triple point of the substance, and it represents the only point in the phase diagram in which all three states are in equilibrium. Point C is the critical point of the substance, which is the highest temperature and pressure at which a gas and a liquid can coexist at equilibrium. The figure below shows what happens when we draw a horizontal line across a phase diagram at a pressure of exactly 1 atm.