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Exciton condensation is collective quantum coherence of electron-hole pairs or excitonic degrees of freedom. A finite exciton population alone does not establish it; the central issue is macroscopic phase coherence or the corresponding ordered state.

The physical setting matters. Long-lived excitons in a semiconductor, electron-hole bilayers and driven exciton-polariton systems have different conservation laws, decay channels and interactions. A spectral feature or an increase in emitted intensity therefore needs supporting coherence and dynamical evidence before being interpreted as condensation.^[ref]

A population of excitons can approach a thermal distribution only if redistribution and cooling occur before too many excitons disappear. A useful necessary scale comparison is ${\tau }_{\mathrm{therm}}\ll {\tau }_{\mathrm{life}}$, where the lifetime includes radiative and nonradiative recombination and the thermalization time describes the relevant energy redistribution.

The inequality is not sufficient to establish exciton condensation: density, disorder, pumping and the stability of the excitons also matter. A continuously pumped short-lived population can have a stationary distribution without satisfying equilibrium statistical mechanics. Spectral narrowing or a nonlinear emission threshold can therefore support several interpretations. Relating such observations to condensation requires a model of creation, cooling, interactions and losses, as emphasized in exciton condensation evidence and competing optical signatures.