surplus value

is new value extracted as profit like a miner extracts diamonds from a mine, which is created by workers in excess of their own labor-cost.

According to Marx's theory, surplus value is equal to the new value created by workers in excess of their own labour-cost, which is appropriated by the capitalist as profit when products are sold.[2][3]' (WIKI)

is value under the aspect of the strange fact that it is possible constantly to sell dearer than one has bought, even on the hypothesis that equal values are always exchanged for equal values.

Whence comes this surplus-value' It cannot come either from the buyer buying the commodities under their value, or from the seller selling them above their value. For in both cases the gains and the losses of each individual cancel each other, as each individual is in turn buyer and seller. Nor can it come from cheating, for though cheating can enrich one person at the expense of another, it cannot increase the total sum possessed by both, and therefore cannot augment the sum of the values in circulation. (. . . ) This problem must be solved, and it must be solved in a purely economic way, excluding all cheating and the intervention of any force the problem being: how is it possible constantly to sell dearer than one has bought, even on the hypothesis that equal values are always exchanged for equal values''[4]

surplus value, n.All

surplus value