The temperature and pressure at which the vapor, liquid, and solid phases of a substance are in thermodynamic equilibrium. The triple point of water, for example, occurs at 273.16 K and 611.7 Pa. At this point, and no other, it is possible to change all of the substance to ice, water, or vapor by making infinitesimally small changes in pressure and temperature. The triple point of water is used to define the kelvin, the SI base unit of thermodynamic temperature.
