The marginal efficiency of capital (MEC) is that rate of discount which would equate the price of a fixed capital asset with its present discounted value of expected income.
The term “marginal efficiency of capital” was introduced by John Maynard Keynes in his General Theory, and defined as “the rate of discount which would make the present value of the series of annuities given by the returns expected from the capital asset during its life just equal its supply price”.