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 p!(p−3)! 3! triple points. Example: In the sulfur system four possible triple points (one metastable) 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]
Citation: 'triple point' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.T06502 RIS BibTex EndNote
Div. IIPDFTextXMLJSON
Last revised: February 24, 2014