Part of total production with which capital buys labor power and which tends to equal the social cost of reproducing it with the necessary skill and knowledge required in production.
It is a part of capital, the part capital uses to buy labor power:
The wage is not the worker's share in the commodity he produces. The wage is the part of already existing commodities with which the capitalist buys a certain amount of productive labor power.
"Wage Labor and Capital", Karl Marx 1849
As the value of labor power, the wage represents the social cost of reproducing the commodity consumed by the capitalist in the productive process. This includes not only the cost of keeping the proletariat alive, but also the cost of maintaining it with the average qualification necessary for its labor power to be productively employed for capital.