Microcanonical ensemble

Summary

The microcanonical ensemble describes an isolated system whose energy, volume and particle number are fixed. It gives equal probability to every microstate with the prescribed energy. Its entropy is $k_B \ln W$, where $W$ counts or measures these microstates and $k_B$ is the Boltzmann constant. The other equilibrium ensembles can be derived from it by looking at a small part of a large isolated system.