kelvin

The kelvin (symbol K) is the SI base unit of thermodynamic temperature. It provides the absolute temperature scale used in science and engineering, with 0 K defined as absolute zero, the point at which molecular motion is minimal.

The kelvin is one of the seven SI base units and is independent of any material substance or physical artifact. It is used for precise measurements of thermodynamic temperature and in equations of state and thermodynamics.

Since 2019, the kelvin is defined by fixing the numerical value of the Boltzmann constant, k, exactly 1.380649×10^-23 J·K^-1. This redefinition ties the unit to a fundamental constant. The Celsius scale is offset from the Kelvin by 273.15, so T(K) = T(°C) + 273.15.

The kelvin retains the historical link to the triple point of water in calibrations, where the triple point temperature is 273.16 K, but this fixed value is no longer the defining standard.

Etymology: the unit is named after Lord Kelvin, William Thomson, a British physicist and engineer who contributed to thermodynamics.