A mathematical way to describe the relationship between the quantity of inputs used by a firm and the quantity of output it produces with them. If the amount of inputs needed to produce one more unit of output is less than was needed to produce the last unit of output, then the firm is enjoying increasing returns to scale (or increasing marginal product). If each extra unit of output requires a growing amount of inputs to produce it, the firm faces diminishing returns to scale (diminishing marginal product).