The minimum efficient scale (MES) is the lowest level of output at which a firm can achieve the minimum average cost of production. It is the point on the long-run average cost curve where the minimum average cost of production is achieved and the curve begins to flatten out. A firm operating at the minimum efficient scale is said to be operating at its most efficient level of production, as it is producing at the lowest possible cost per unit.

The minimum efficient scale is an important concept in economics because it helps to determine the optimal size of a firm and the level of competition in an industry.