In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium.
For example, the triple point temperature of water is exactly 273.16 kelvin (0.01°C) and the triple point pressure is 611.73 pascals (ca. 6 millibars). The triple point temperature of mercury is -38.8344°C.
The triple point of water is used to define the kelvin, the unit of thermodynamic temperature in the International System of Units. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.
Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.