The only difficulty about the word "labor" lies in the ambiguity of the word "work." Everyone understands that when we talk about laborers, we are referring to those who work for a living. Lacking property in the means of production (i.e. capital) they are reduced to toiling for the wages, or salaries that they receive for their work.
If high-salaried executives have no source of income other than their salaries, and if they would not work unless they were paid for what they do, they, too, are toilers, similar to the least skilled workers who would not be working except for the extrinsic compensation they receive for what they are engaged in doing. Strictly speaking, in Marxist terms, they all belong to the proletariat -- the toilers who would not work if they did not need the wages or salaries they receive.
Toil has no intrinsic value for those who do it, unless it be the avoidance of stealing as a way of obtaining what they need in order to survive. But there is another meaning of the word "work" that is not toil. Let us call toil subsistence-work. The other meaning of "work" is that which anyone would do if one did not need income for doing it. Because it is work worth doing for its own sake for its intrinsic value.
For example, composing music, painting a portrait, teaching any activity that perfects the individual who does it, by improving or perfecting himself or herself, may be as tiring as toil, but we are willing to do such work because of the benefits it confers upon us or upon society. Let us call such work leisure-work. It is leisure-work even when it is compensated as well as when it is done without any thought of extrinsic compensation.
I would not use the words "human capital" for any member of the working class. In my view, the opposition of labor and capital must preserve the distinction between labor and all means of production and all consumable goods and services. Capital can be owned, but labor cannot be, except by the laborer himself or herself. Chattel slavery is a violation of this basic truth.
Unfortunately, the word "leisure" is generally misused as if it were a synonym for time that is free from subsistence work, from leisure work, and from other things that are needed to stay alive and healthy.
It is also used as if it referred to recreation or play. (See Leisure.) Leisuring is an activity as much as toiling is.
If we remember that there are two forms of work -- toiling or subsistence-work and leisure-work -- we will never again use the word "leisure" for recreations that comprise all the forms of playing, or for just doing nothing.
Adler's Philosophical Dictionary (1995)
Great Ideas from the Great Books (1963)
by Mortimer J. Adler