Other forms: kelvins
A kelvin is a unit for measuring temperature. The kelvin scale is based on absolute zero, the coldest possible measurement, so there is no such thing as a negative temperature in kelvins.
The Celsius scale is based on kelvins, but the two scales differ in that kelvins measure an absolute temperature, based on a zero that can literally not get any colder. While temperature isn't commonly measured in kelvins outside the science lab, it's a vital way of calculating temperature for scientists. The kelvin is named after the Irish-Scottish physicist Sir William Thompson, also known as Lord Kelvin.