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, the labor power produces the value equivalent to its reproduction, i.e., the wage. In the second, labor power generates surplus value. Surplus labor is therefore the measure in social time, i.e. in average necessary social time, of surplus value. The secret of the process of capital valorization is none other than the capacity to obtain unpaid work.
The capitalist pays for necessary labor the value of labor power (or its price, divergent from its value) and in return he obtains the right to use living labor power as he wishes. His use of this labor power is broken down into two periods.
During one of these periods the worker produces only one value = the value of his labor power, that is, only one equivalent. In exchange for the advanced price of the labor power, the capitalist, in this way, obtains a product of the same price. It is as if he had acquired 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 a substitute for value. He obtains this mobilization of labor power for free. It is in this sense that surplus labor can be called unpaid labor.
Capital, therefore, is not only the possibility of disposing of labor, as Adam Smith says. It is, in essence, the possibility of disposing of unpaid labor. Any surplus value, whatever the particular figure - profit, interest, rent, etc. - in which it subsequently crystallizes, is, according to its substance, the material concretion of unpaid labor time. The mystery of capital's self-valorization is solved by the fact that it can dispose of a certain amount of unpaid foreign labor.
Karl Marx. Capital, Book I, Chapter XVI, 1867.