(symbol k), a fundamental constant of physics occurring in nearly every statistical formulation of both classical and quantum physics. The constant is named after Ludwig Boltzmann (Boltzmann, Ludwig Eduard), a 19th-century Austrian physicist, who substantially contributed to the foundation and development of statistical mechanics, a branch of theoretical physics. Having dimensions of energy per degree of temperature, the Boltzmann constant has a value of 1.380650 × 10⁻²³ joule per kelvin (K), or 1.380650 × 10⁻¹⁶ erg per kelvin.

The physical significance of k is that it provides a measure of the amount of energy (i.e., heat) corresponding to the random thermal motions of the molecules of a substance. For a classical system at equilibrium at temperature T, the average energy per degree of freedom is kT/2. In the simplest example of a gas consisting of N noninteracting atoms, each atom has three translational degrees of freedom (it can move in the x-, y-, or z-directions), and so the total thermal energy of the gas is 3NkT/2.