  The Boltzmann entropy, SB = k lnW,  is widely regarded as being the most natural analog within statistical mechanics for the thermodynamic entropy. It is a property of an individual microstate. The state space is divided up into a number of distinct regions and SB is defined in  terms of the volume, W, of the region of state space to which the microstate belongs. All microstates within a given region have the same Boltzmann entropy.