In uncountable|lang=en terms the difference between instability and instability

is that instability is (uncountable) the quality of being unstable while instability is (uncountable) the quality of being unstable.

In physics|countable|lang=en terms the difference between instability and instability

is that instability is (physics|countable) a state that is not in equilibrium, or in which a small change has a large irreversible effect while instability is (physics|countable) a state that is not in equilibrium, or in which a small change has a large irreversible effect.

As nouns the difference between instability and instability

is that instability is (uncountable) the quality of being unstable while instability is (uncountable) the quality of being unstable.