The kelvin is a unit of measurement for temperature in the International System of Units (SI). It is named after the Scottish physicist William Thomson, 1st Baron Kelvin, who contributed to the understanding of thermodynamics. The kelvin scale starts at absolute zero, which is 0 K, the point where all molecular motion ceases. Unlike the Celsius and Fahrenheit scales, the kelvin scale does not use degrees. Instead, it measures temperature in kelvins (K). One kelvin is equivalent to one degree Celsius, but the two scales have different starting points. For example, 0 °C is equal to 273.15 K.