A metastable state is locally stable against small disturbances but has a higher thermodynamic potential than another accessible phase. In a free-energy landscape it occupies a local minimum separated from the stable state by a barrier.
A liquid undergoing supercooling illustrates the distinction: small fluctuations can relax without freezing, but a sufficiently large crystal nucleus can grow. The nucleation barrier depends on the competition between interfacial cost and bulk free-energy gain. Waiting time is therefore affected by sample volume, impurities, surfaces and observation duration. A spinodal point is the mean-field limit where the metastable minimum loses local stability; it is distinct from the coexistence condition where the stable phases have equal appropriate intensive potentials. Metastability explains hysteresis without making the equilibrium phase boundary path dependent.
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