metastable

adjective

1. Metallurgy. chemically unstable in the absence of certain conditions that would induce stability, but not liable to spontaneous transformation.
2. Also labile. Physics, Chemistry. pertaining to a body or system existing at an energy level (metastable state) above that of a more stable state and requiring the addition of a small amount of energy to induce a transition to the more stable state.

adjective

1. (of a body or system) having a state of apparent equilibrium although capable of changing to a more stable state
2. (of an atom, molecule, ion, or atomic nucleus) existing in an excited state with a relatively long lifetime

noun

3. a metastable atom, ion, molecule, or nucleus