Consider a system which is in an equilibrium state. Now, a small perturbation causes it to start oscillating about the equilibrium state, but over time, the momentum with which it overshoots the equilibrium state keeps on increasing. This can be considered as an exact opposite of what can be expected of reaching a normal stable equilibrium state (frictional forces reducing excess momentum until equilibrium is reached).
What is such kind of an equilibrium state called? Is it still referred to as a stable equilibrium since we keep oscillating about the state?