Time of use of labor power that exceeds the time needed to produce the value equivalent to its cost of reproduction.
Surplus labor and surplus value
The production process can be divided into two parts. In the first, labor power produces the value equivalent to its reproduction, that is, the wage. In the second, labor power generates surplus value. Surplus labor is therefore the measure in social time, that is, in average socially necessary time, of surplus value. The secret of the process of valorization of capital is none other than the capacity to dispose of unpaid labor.
The capitalist pays the value of labor power (or its price, diverging from its value) and in exchange obtains the right to dispose of living labor power. His use of this labor power is divided into two periods.
During one of these periods the worker produces only a value = the value of his labor power, that is, only an equivalent. In exchange for the advanced price of labor power, the capitalist thus obtains a product of the same price. It is as if he had bought the finished product on the market. In the period of surplus labor, on the contrary, the use of labor power forms value for the capitalist without that value costing him any replacement of value. He obtains this mobilization of labor power for nothing. It is in this sense that surplus labor can be called unpaid labor.
Capital, therefore, is not only the command over labor, as Adam Smith says. It is, in essence, the command over unpaid labor. All surplus value, whatever particular form -profit, interest, rent, etc.- it may subsequently crystallize into, is in substance the materialization of unpaid labor time. The mystery of the self-valorization of capital resolves itself into the fact that capital can dispose of a definite quantity of other people's unpaid labor.
Karl Marx. Capital, Book I, Chapter XVI, 1867.