Kelvin

The Kelvin temperature scale is the most commonly used absolute temperature scale in the world.

The kelvin scale differs from the more familiar Celsius or centigrade (℃) temperature scale; there is no such thing as a below-zero Kelvin figure. A temperature of 0 K represents absolute zero, the absence of all heat. However, the size of the kelvin "degree" is the same as the size of the Celsius "degree." A change of plus-or-minus 1℃ is the same as a change of plus-or-minus 1 K.

In 2018, the Kelvin was redefined to make it more accurate. The new definition is: "The kelvin, symbol K, is the SI unit of thermodynamic temperature; its magnitude is set by fixing the 
numerical value of the Boltzmann constant to be equal to exactly 1.380649 × 10^-23 J K^-1 [joules per kelvin]."

At standard Earth-atmospheric sea-level pressure, water freezes at 0℃ or +273.15 K, and boils at +100℃ or +373.15 K. A temperature of 0 K thus corresponds to -273.15℃.