Capital is wealth that is used in production, or wealth that is in the course of exchange.
That seems clear enough at first, but of all the classical definitions, it is the most readily forgotten and misconstrued. It might be helpful to remember some of the things that capital is not. Capital is not money, or stock certificates, or bonds, because those are things that do not directly satisfy desires. It is not land, because land is not produced by labor. It is not any form of service or skill, because capital is the material product of labor, not the labor itself.