Triple Point

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. At 'Triple Point', the sublimation curve, fusion curve, and the vaporization curve of a substance meet together.

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, like Helium.

The triple point of water was used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI). The term "triple point" was coined in 1873 by James Thomson, brother of Lord Kelvin.