Relative surplus value
Amount of surplus value extracted from the workforce per unit of time. The increase in surplus value in relative terms, i.e. the rate of surplus value, occurs through increases in labor productivity associated with technical improvements and increases in invested capital.
The rate of surplus value is the measure of relative surplus value.
Rate of surplus value = surplus value / variable capital
Definition in "Capital"
In "Capital", Marx defines surplus value based on the relationship between the two parts of the day: that in which the labor force generates the value equivalent to its own reproduction and that in which it generates surplus value.
I call absolute surplus value that which is produced by extending the working day; on the contrary, that which arises from the reduction of the necessary working time and the consequent change in the proportion of the magnitude of the two components of the working day, I call relative surplus value.
Karl Marx. Capital, Book I, Chapter X, 1866.
Relative surplus value in capitalist development
The development of exploitation in relative terms was the most important element of progress of the productive forces of rising capitalism.
By incorporating technologies that allowed to produce more with less working hours, that is, to increase relative exploitation, capitalism could raise wages and increase profits at the same time... but only if the market increased as well.
That was the driving force behind capitalism all over the world during the 19th century. For a century now, there are no longer any "virgin" markets, the system has entered its historical decline. The inevitable consequence is that technological improvement tends to increase unemployment and precarization.http://dictionary.marxismo.school/Relative surplus value