What characterizes an unstable equilibrium in a system?

An unstable equilibrium in a system is characterized by the ability to form a new equilibrium after experiencing a disturbance. In this scenario, when a system at an unstable equilibrium is disturbed, it does not return to its original state but instead shifts to a different equilibrium position. This behavior is indicative of systems that are sensitive to external influences, where small changes can lead to significant and often irreversible shifts.

For example, in ecological systems, if a population is at an unstable equilibrium due to environmental changes or resource depletion, any disturbance can push the population into a new equilibrium state, possibly leading to a complete change in the ecosystem dynamics, such as a shift from one dominant species to another.

In contrast, other options describe scenarios where the system either returns to its original state, remains unchanged despite disturbances, or maintains equilibrium indefinitely, which are characteristics of stable or neutral equilibria, not unstable ones.