Metastable

Metastable is a state in which matter experiences a local equilibrium, but will be disturbed by a small perturbation. The metastable state is at a delicately equilibrium which will remain unless it is disturbed after which it will proceed to a more stable equilibrium with lower energy.

Figure 1. E vs. x showing stable and metastable states.

References

http://en.wikipedia.org/wiki/Metastable