A classical system is metastable if it
is above its minimum-energy state, but requires an energy input
before it can reach a lower-energy state; accordingly, a metastable
system can act like a stable system, provided that energy inputs (e.g., thermal fluctuations) remain below some threshold. Systems with
strong metastability are commonly described as stable. Quantum mechanical
effects can permit metastable states to reach
lower energies by tunneling, without an energy input; an associated, broader definition of metastable embraces all systems that have a long lifetime (by some standard) in
a state above the minimum-energy state.