The particle number, N, is the number of so called 'elementary particles' (or elementary constituents) of a thermodynamical system. By calling a constituent element, we state that it cannot be broken into smaller pieces at the scale of energy k·T involved in the process. Our definition of elementary particle is thus temperature dependent.

where

k is the Boltzmann constant, measured in joules per kelvin

T is the temperature, measured in kelvins