Reserve army of labour

In Das Kapital, Volume I, Chapter 25, Section 3, titled "Progressive Production of a Relative surplus population or Industrial Reserve Army", Karl Marx develops the idea of an reserve army of labour, also sometimes called an industrial reserve army, or reserve army of unemployed, or relative surplus population. Marx did not invent the term "reserve army of labour". It was already being used by Friedrich Engels in his 1845 book The Condition of the Working Class in England. What Marx did was theorize the reserve army of labour as a necessary part of the capitalist organization of labour-power.