The basic SI unit of thermodynamic temperature, defined as 1/273.16 of the thermodynamic temperature of the triple point of water, i.e. the equilibrium point at which pure ice, air-free water, and water vapour can coexist in a sealed vacuum flask. It is related to the degree Celsius (°C):
where T is the thermodynamic temperature and 273.15 K is the temperature of the ice point, which is the zero of the Celsius (or centigrade) temperature scale.
The kelvin is also used as a unit of temperature difference, in which case it is identical to the degree Celsius.