What is the name of the point on a phase diagram where three phases coexist?

The point on a phase diagram where three phases coexist is called the triple point.

For the familiar solid–liquid–gas case, the three phases exist together in thermodynamic equilibrium at one specific combination of temperature and pressure. On a standard phase diagram, the sublimation, fusion and vaporization curves meet at this point.

Water’s triple point occurs at approximately 273.16 K and 611.657 pascals, where ice, liquid water and water vapor coexist. Before the 2019 SI revision, 273.16 K was used to define the kelvin; the kelvin is now defined through the Boltzmann constant, although water’s triple point remains an important calibration reference.

The triple point is not the same as a melting point or boiling point. Those terms usually describe a phase boundary at a specified pressure, while a triple point requires three phases simultaneously. A critical point is different again: it marks the end of a liquid–gas coexistence curve, beyond which the two phases become indistinguishable. Substances with multiple crystal structures can also have triple points involving three solid phases.