This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

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

adj. 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.

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.

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

"This spring (a period of great disappointment for our farmers) was a period of the so-called metastable conditions of rain clouds."

subgame-perfect    neither-painful-nor-pleasant    no-fly    100-101    long-period