Kelvin is a unit of measurement used to quantify the color temperature of light sources. It is the SI unit of thermodynamic temperature and is denoted by the symbol K. The kelvin scale is an absolute scale that starts at 0 kelvin, representing absolute zero, which is the point at which a thermodynamic system has its lowest energy. The kelvin scale is different from the Fahrenheit and Celsius temperature scales as it is not expressed in degrees and does not have negative values.