The Kelvin temperature scale (also referred to as the Thermodynamic temperature scale) are measured not in Degrees but in units called "Kelvins" (K). They are so named in honour of Lord William Thomson Kelvin (1824-?). However, the thermodynamic scale was defined so that a Kelvin was exactly the same size as a degree Celcius.
The scale was developped by extrapolating backwards on "volume-temperaturte" and "pressure-temperature" graphs. Using such extrapolation it was discovered that the temperature axis was cut at -273 Degrees C. This suggested that this was then the lowest temperature attainable and consequently this was called the absolute zero. Thus by shifting the vertical axis 273 Degrees C to the left and renumbering the temperature scale, -273 Degrees C became 0K, the ice point 273K and the steam point 373K.
(Lord Kelvin)