What is meant by minimum efficient scale?

The minimum efficient scale is the scale of operation of the smallest firm in the industry.
The minimum efficient scale identifies the method by which a firm can produce a given level of output with minimum inputs.
The minimum efficient scale is the minimum quantity that the firm can produce given the level of inputs.
The minimum efficient scale is the size of operation with the lowest average cost.

Explanation
The minimum efficient scale is the minimum size a firm must be in order to achieve the lowest average total costs.