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Ionization equilibrium means that the rates populating and depopulating ionization stages balance under a specified physical model. For a neutral species photoionized at rate $\Gamma$ per atom and singly charged ions recombining with coefficient $\alpha$, a simple balance is $n_0\Gamma =n_e{n}_{+}\alpha$.

This kinetic balance need not be local thermodynamic equilibrium. Collisional ionization and radiative recombination can balance in a thin plasma that does not obey a Saha population law. Moreover, a rapidly cooling or flowing gas may retain a nonequilibrium ionization state if its ionization or recombination times exceed the evolution time. A temperature alone is therefore insufficient to declare ionization equilibrium.

Jelle S. Kaastra develops the physical interpretation of X-ray radiation spectra from hot astronomical plasma. His review with collaborators explains how ionization equilibrium, spectral lines and continuum radiation constrain the diffuse gas in galaxy clusters and between galaxies.

A spectrum acts as a diagnostic because different ions and transitions respond differently to temperature and other physical conditions. The interpretation requires an atomic model: an observed spectral line is evidence about the emitting plasma, rather than a direct reading of a single universal temperature.^[ref]