metastable

1. Of uncertain stability; in a condition to pass into another phase when slightly disturbed, for example, water, when cooled below the freezing point may remain liquid but will immediately congeal if a piece of ice is added.

2. Denoting the excited condition of the nucleus of a radionuclide isomer that reaches a lower energy state by the process of isomeric transition decay without changing its atomic number or weight; for example, ^99m₄₃Tc → ⁹⁹₄₃Tc + γ.