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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 kelvin 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 (0.01 °C). The Celsius temperature scale is now defined in terms of the kelvin.

The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin.

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 scales of measurement, whereas the kelvin is a unit of measurement. 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. Unlike temperature scales which use the degree symbol, there is a space between the number and the K, as with all other SI units.

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