The kelvin
The kelvin (K) is the SI base unit of thermodynamic temperature. Since the 2019 SI redefinition it is defined by fixing the Boltzmann constant to exactly 1.380649 × 10⁻²³ J/K. Zero kelvin is absolute zero, the lowest temperature theoretically possible. The kelvin is preferred in science and engineering because it has a true zero — temperature ratios become physically meaningful, unlike on the Celsius or Fahrenheit scales. Note: by convention there is no degree sign — 300 K, not 300 °K.