marginal product or marginal physical product is the extra output produced by one more unit of an input (for instance, the difference in output when a firm’s labour is increased from five to six units).

Assuming that no other inputs to production change, the marginal product of a given input X can be expressed as

\mathrm{MP} =  \frac{\Delta \mathrm{Y}}{\Delta \mathrm{X}}

where ΔX is the change in a firm’s production inputs and ΔY is the change in quantity of production output.