Macrostate

A macrostate of a thermodynamic system is a specification in terms of macroscopic variables such as temperature $T$, pressure $P$, volume $V$, and particle number $N$ (or chemical potential $\mu$).

Relation to microstates

Many distinct microstates typically correspond to the same macrostate. The Boltzmann entropy quantifies this multiplicity:

where $\Omega$ is the number of microstates (or phase space volume) compatible with the given macrostate.

In equilibrium statistical mechanics

The macrostate is characterized by the values of state functions . Different ensembles (microcanonical, canonical, grand canonical) correspond to different choices of which macroscopic variables are held fixed.