The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined by two facts: zero kelvins is absolute zero (when molecular motion stops), and one kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The Celsius temperature scale is now defined in terms of the kelvin, with 0 °C corresponding to 273.15 kelvins, approximately the melting point of water under ordinary conditions.
The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin.
In Unicode, a legacy code for a kelvin symbol (K) to accommodate some old code pages in certain Oriental languages exists; it is not recommended for use any more. In all languages, the symbol should be the Roman letter Unicode K for current usage.
The word kelvin as an SI unit is correctly written with a lowercase k (unless at the beginning of a sentence), and is never preceded by the words degree or degrees, or the symbol °, unlike Fahrenheit, or Celsius. This is because the latter are adjectives, whereas kelvin is a noun. It takes the normal plural form by adding an s in English: kelvins. When the kelvin was introduced in 1954 (10th General Conference on Weights and Measures (CGPM), Resolution 3, CR 79), it was the "degree Kelvin", and written °K; the "degree" was dropped in 1967 (13th CGPM, Resolution 3, CR 104).
Note that the symbol for the kelvin unit is always a capital K and never italicised. There is a space between the number and the K, as with all other SI units.
In a thermodynamic system, the energy carried by the particles is proportional to the absolute temperature, where the constant of proportionality is the Boltzmann constant. As a result, it is possible to determine the temperature of particles with a certain energy; or to calculate the energy of particles at a certain temperature:
electron volts to kelvins
kelvins to electron volts
ITS-90 International Temperature Scale