The SI unit of temperature is Kelvin, which is a six-letter word. Kelvin is a unit of measurement for temperature that is part of the International System of Units (SI). It is named after the British physicist and engineer William Thomson, 1st Baron Kelvin, who proposed it in 1848. The Kelvin scale is an absolute thermodynamic temperature scale where zero is defined as absolute zero, the temperature at which all thermal motion ceases.