(generally)
Anything which is used to procure or extract surplus value.
Capital is not a static definition, but rather constitutes an economic relation. Machinery that forms the instruments of production, such as industrial machinery and property that is a condition of production, such as farmland or the physical fabric of a factory, are both forms of capital (see means of production, the), money that is used to set labor in motion is all capital. Money used to buy food and so forth is not capital, unless that food is being used to pay laborers.
Capital is any material investment of any kind that results in the investor receiving more in return than they invest.
Marx gives us the equation M-C-M’, that is (money) is exchanged for -> (commodity) is exchanged for -> (money with increased value, i.e. initial money + surplus value).
Any money or object (including means of production) used to gain money prime (money increased) is in or performing the capital relation, whether it is in the form of liquid currency or not.
(metonymically)
A shorthand for capitalists and all of their agents. (Thus, “capital is the bastion of reaction”).