  Another novelty is that Boltzmann has changed his concept of equilibrium. Whereas previously the essential characteristic of an equilibrium state was always that it is stationary, in Boltzmann's new view it is conceived as the macrostate (i.e., a region in phase space) that can be realized in the largest number of ways. As a result, an equilibrium state need not be stationary: in the course of time, the system may fluctuate in and out of equilibrium.