The Kelvin (K) is the standard unit of thermodynamic temperature in the International System of Units (SI). As an absolute temperature scale, it measures the average kinetic energy of the particles within a system. Zero Kelvin represents absolute zero, the theoretical temperature at which particle motion ceases. The scale is fundamental to thermodynamics and physical chemistry, providing a direct, linear relationship to temperature and directly utilized in calculations involving entropy and the gas constant. It is particularly valuable in scientific contexts because its value is directly proportional to the internal energy of the substance, unlike other common scales that do not originate at a true zero point.