Wiktionary
n. (context physics English) The unique temperature and pressure at which the solid, liquid and gas phases of a substance are all in equilibrium.
Wikipedia
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases ( gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. For example, the triple point of mercury occurs at a temperature of −38.83440 °C and a pressure of 0.2 m Pa.
In addition to the triple point for solid, liquid, and gas phases, a triple point may involve more than one solid phase, for substances with multiple polymorphs. Helium-4 is a special case that presents a triple point involving two different fluid phases ( lambda point). pure oxygen.
The triple point of water is used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI). The value of the triple point of water is fixed by definition, rather than measured. The triple points of several substances are used to define points in the ITS-90 international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K, 0.01 °C, or 32.018 °F).
The term "triple point" was coined in 1873 by James Thomson, brother of Lord Kelvin.
Usage examples of "triple point".
Water at the triple point is ice, liquid, and vapor at the same time.
They had now only to descend the mountain slopes again, and explore the soil, in the triple point of view, of its mineral, vegetable, and animal resources.