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
"triple point." YourDictionary, n.d. Web. 11 October 2018. <http://www.yourdictionary.com/triple-point>.
triple point. (n.d.). Retrieved October 11th, 2018, from http://www.yourdictionary.com/triple-point
The temperature and pressure at which a substance can exist in equilibrium in the liquid, solid, and gaseous states. The triple point of pure water is at 0.01 degrees Celsius and 4.58 millimeters of mercury and is used to calibrate thermometers.
This comparison with fusion introduces a second notion, that of the "triple-point," this being in the melting-phenomenon the only temperature at which solid, liquid and vapour are in equilibrium, in other words, where three phases of one substance are co-existent.