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 metastable) exist for the four phases comprising rhombic $\ce{S}$ (solid), monoclinic $\ce{S}$ (solid), $\ce{S}$ (liquid) and $\ce{S}$ (vapour).