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.
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.
Another example is super-cooled 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.
Metastability in a congested state of a network works differently if the network is congested enough. New calls are unlikely to flow through, and because of this it will require a second circuit which increases congestion further.
Metastable state | chemistry and physics
Supercooled Water - Explained!
March 22, 2011
Supercooled water escaping from metastability