When the addition of successive extra units of any input to a productive process yield smaller increases in output. Consider using an extra unit of one input in a production process while the quantities of other inputs are held constant. If total output increases, the input has a positive marginal product. The input has a diminishing marginal product if output rises at a diminishing rate as successive extra units of the input are used. It is possible for total output to fall as more input is used, in which case marginal product is negative.