Amount of surplus value extracted from labor power per unit of time. The increase in surplus value in relative terms, that is, in the rate of surplus value, comes about through the increase in labor productivity associated with technical improvements and the increase 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 on the basis of the relationship between the two parts of the working day: the one in which labor power generates the value equivalent to its own reproduction and the one in which it generates surplus value.
The surplus-value produced by prolongation of the working-day, I call absolute surplus-value. On the other hand, the surplus-value arising from the curtailment of the necessary labour-time, and from the corresponding alteration in the respective lengths of the two components of the working-day, I call relative surplus-value.
Karl Marx. Capital, Book I, chapter X, 1867.
Relative surplus value in capitalist development
The development of exploitation in relative terms was the most important element of progress of the productive forces in rising capitalism.
By incorporating technologies that made it possible to produce more with fewer hours of work, that is, to increase relative exploitation, capitalism could raise wages and increase profits at the same time... although only on condition that the market grew as well.
That was the driving force behind the expansion of capitalism all over the world during the 19th century. For a century now there have been no "virgin" markets left; the system has entered its historical decadence. The inevitable consequence is that technological improvement tends to increase unemployment and precarization.