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 (https://doi.org/10.1351/pac199466030577)