The kelvin (symbol: K) is a unit to measure temperature. It is one of the seven SI base units. It is defined by two factors: 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 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 °, like Fahrenheit, Celsius or centigrade. This is because the latter three are scales of measurement, whereas the kelvin is a unit of measurement.