Unstable equilibrium refers to a state where a system is in balance but is sensitive to small disturbances. If a slight change occurs, the system will move away from this balance rather than return to it. For example, a pencil balanced on its tip is in unstable equilibrium; even a tiny nudge will cause it to fall. In physics, unstable equilibrium is often illustrated using a ball on a hilltop. The ball is at rest at the peak, but any small push will cause it to roll down the slope, moving the system away from equilibrium. This concept is important in various fields, including mechanics and economics.