the temperature and pressure at which the three phases of a substance are in equilibrium. The triple point of water, 273.16 K at a pressure of 611.2 Pa, is the basis of the definition of the kelvin
triple point in American English
a pressure and temperature combination at which the solid, liquid, and vapor phases of a substance exist in contact and in equilibrium with one another: the triple point of water is 273.16°K (0.01°C)at c. 4.6 mm of mercury