A scientific

constant represented by the letter k and used in

thermodynamics and statistical mechanics. Its approximate value is

k = 1.38 * 10

^{-23} J/K.

There is a thermodynamic principle that when a system can take on a number of configurations, the probability of a particular configuration is proportional to e^(-E/kT), where E is the internal energy of the configuration and T is the absolute temperature.

Boltzmann's constant also turns up in these two important physical equations:

- S = k log W (S is entropy, and W is the number of possible configurations of a system)
- PV = NkT. This equation describes the behaviour of an ideal gas. P is the pressure, V is the volume and N is the number of molecules.