triple point

The point in a one-component system at which the temperature and pressure of three phases are in equilibrium. If there are $Math - ei$ possible phases, there are $Math - ei$ 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