In thermodynamics, a system's condition can be described at two levels: macrostates (bulk properties such as pressure, temperature, and volume) and microstates (the exact position and momentum of every constituent particle at an instant). Macrostates are statistical/aggregate descriptions that are only well defined when the system is in thermodynamic equilibrium — meaning these properties are uniform throughout the system and unchanging over time — whereas microstates are always well defined regardless of equilibrium, since they specify particle-level detail directly. This distinction, foundational to statistical mechanics and classical thermodynamics, explains why macroscopic state variables cannot be assigned during non-equilibrium (transient/fluctuating) processes even though the underlying microstate always exists.
In thermodynamics, a system's condition can be described at two levels: macrostates (bulk properties such as pressure, temperature, and volume) and microstates (the exact position and momentum of eve…