thermodynamic temperature), a parameter of state that characterises a macroscopic system in a state of thermodynamic equilibrium (the absolute temperature of all its macroscopic subsystems is then the same). Absolute temperature was introduced in 1848 by the English physicist W. Thomson (Kelvin) on the basis of the second law of thermodynamics. Absolute temperature is denoted by the symbol T, is expressed in kelvins (K) and is counted from absolute zero of temperature. It is measured on the thermodynamic and the international practical temperature scales.