A substance may exist as a solid, liquid, and gas at which point?

The triple point of a substance is the unique set of conditions at which it can coexist in all three phases: solid, liquid, and gas. This point occurs at a specific temperature and pressure, where the three phases are in thermodynamic equilibrium. For example, the triple point of water occurs at 0.01 degrees Celsius and a pressure of 611.657 pascals. At this condition, water can exist simultaneously as ice, liquid water, and water vapor.

In contrast, the other points mentioned relate to transitions between two phases rather than all three. The critical point is where the liquid and gas phases become indistinguishable. The boiling point is the temperature at which a liquid turns to gas, and the freezing point is where a liquid becomes a solid. None of these points allow for the existence of all three states concurrently, which is a defining characteristic of the triple point.