triple point

https://doi.org/10.1351/goldbook.T06502
The point in a one-component system at which the temperature and pressure of three phases are in equilibrium. If there are \(p\) possible phases, there are \(\frac{p!}{(p\,-\,3)!\ 3!}\) triple points. Example: In the sulfur system four possible triple points (one @M03872@) exist for the four phases comprising rhombic S (solid), monoclinic S (solid), S (liquid) and S (vapour).
Source:
PAC, 1994, 66, 577. (Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)) on page 593 [Terms] [Paper]