Unstable equilibrium refers to a state where a system is in balance but is sensitive to small disturbances. In this condition, if the system is slightly disturbed, it will move away from its original position rather than return to it. This concept is often illustrated using a ball placed on top of a hill; a slight push will cause it to roll down, leading to a new state. In contrast to stable equilibrium, where systems return to their original state after disturbances, unstable equilibrium can lead to significant changes. This principle is applicable in various fields, including physics, economics, and ecology, where systems can shift dramatically due to minor changes in conditions.