metastable

Of, relating to, or being an unstable but relatively long-lived state of a chemical or physical system, as of a supersaturated solution or an excited atom.

In phys. ehem., having a stability of such sort that a minute impressed change of conditions may produce a disturbance not proportional to the impressed change.

In thermodynamics, in a state intermediate between stable equilibrium and unstable equilibrium, but approaching stability as nearly as the conditions will permit.

Of or pertaining to a physical or chemical state that is relatively long-lived, but may decay to a lower energy state when slightly perturbed or through a quantum transition.

(of physical systems) continuing in its present state of equilibrium unless sufficiently disturbed to pass to a more stable state of equilibrium