Boltzmann constant, one of the fundamental physical constants; it is equal to the ratio of the gas constant R to the Avogadro constant NA and is denoted by k; it is named in honour of the Austrian physicist L. Boltzmann. The Boltzmann constant enters a number of the most important relations of physics: the equation of state of an ideal gas, the expression for comparing the energy of the thermal motion of particles; it relates the entropy of a physical system to its thermodynamic probability. The Boltzmann constant k = 1.380662(44)·10-23 J/K (as of 1980). This value was obtained from the data on R and NA. The value of the Boltzmann constant can be determined directly, for example, from the experimental verification of the laws of thermal radiation.