How did Marx define surplus value in capitalist production?

In Marxist economic theory, Karl Marx defined surplus value as the difference between the total value created by a worker and the portion of that value they receive as wages.12

Adhering to the labour theory of value, Marx argued that human labour is the sole source of economic value. In a capitalist system, the capitalist purchases a worker's labour power, paying only enough to cover the worker's means of subsistence. However, the worker is compelled to work for a duration longer than what is required to produce the equivalent of their own wages.123

The value produced during this additional time is "surplus labour," which manifests as surplus value. Because the capitalist owns the means of production, they appropriate this surplus value, which Marx identified as the fundamental mechanism of exploitation in capitalist production. Marx viewed this appropriation as the source of profit and a primary driver of the systemic instability and contradictions inherent in capitalism.123