In physics and chemistry, a system is said to be in a metastable state if it has a higher energy than at its grounded state and it can be stuck in that higher-energy state for an unknown/indefinite period of time.
Example #1: Ball on a hilltop
For example, a ball that is sitting on a hilltop and not accelerating is in a metastable state. The ball would have less energy if it was resting at the bottom of the hill on either side, but it won't reach that state unless some force acts on it, such as a gust of wind or a push.
Example #2: Supercooled water
Another example is supercooled water, which can exist in a liquid metastable state even at temperatures below its freezing point until a nucleation event occurs so that the water molecules can crystallize and transform into ice.
Metastable state | chemistry and physics
Supercooled water escaping from metastability
Documentaries, videos and podcasts
Supercooled Water - Explained!
March 22, 2011