The triple point of a substance is a term used in thermodynamics, representing the intersection on a phase diagram, where three phases of matter (gas, liquid, and solid) coexist in equilibrium. As an example, the triple point of mercury occurs at a temperature of −38.8344 °C and a pressure of 0.2 mPa.
There is not only a triple point between solid, liquid, and gas, as we can add triple points involving more than one solid phase, for substances with multiple polymorphs. A special case of this type is Helium-4, which has a triple point involving two different fluid phases.
In general, for a system with p possible phases, there are triple points.
The triple point of water is used to define the Kelvin. The triple point temperature of water is an exact definition and not a measured quantity (the triple point of pure water is at 0.01 degrees Celsius and 4.58 millimeters of mercury and is used to calibrate thermometers). Many substances’ triple points are used to mark points in the ITS-90 (International Temperature Scale of 1990), varying from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K).