Triple Point

The Triple Point is the temperature at which a substance exists in all three phases, solid, liquid and gas in thermodynamic equilibrium.  For instance, the triple point of water is 273.16 K or .016 deg. C

Triple points are useful in calibrating thermometers.  The temperature of a pure substance is adjusted such that all three phases are in equilibrium.  This temperature, the triple point, is then used to calibrate the thermometers.