triple point
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).
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 593 (