Long-run equilibrium is a state in a market where firms have had enough time to enter or exit, resulting in no economic profits or losses, and where the quantity supplied equals the quantity demanded. In this state, firms produce at an output level where their average total costs are minimized, and consumers are satisfied with the price and quality of the goods available. This concept is crucial for understanding how different market structures function over time, particularly in the dynamics of competition and the ability for firms to adjust their operations.