Labor-power

As described by Karl Marx in Vol 1, Chapter 6 of Capital:

"By labor-power or capacity for labor is to be understood the aggregate of those mental and physical capabilities existing in a human being, which he exercises whenever he produces a use-value of any description. "

Or in simpler terms, labor-power is the capacity to do work. Capitalists pay wages to workers in exchange for their capacity to work.

Notably, Marx & Engels revised the Communist Manifesto to use the term 'selling of labor-power' instead of the 'selling of labor'.