what is the triple point of a substance

The triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium.

For example, the triple point temperature of mercury is at −38.8344 °C, at a pressure of 0.2 MPa.

The triple point of water is used to define the kelvin, the SI base unit of thermodynamic temperature. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.