What is Capital Stock?

Legal Definition
Capital stock is the combination of a corporation's common stock and preferred stock (if any). Common stock is usually the first and only capital stock issued by corporations. However, some corporations will also issue preferred stock. The amount received by the corporation when it issued shares of its capital stock is reported in the stockholders' equity section of the balance sheet.
Legal Definition
The common stock or fund of a corporation. The sum of money raised by the subscriptions of the stockholders, and divided into shares. It is said to be the sum upon which calls may be made upon the stockholders, and dividends are to ba paid. Christensen v. Eno, 106 N. Y. 97, 12 N. E. 648, 60 Am. Rep. 429; People v. Ooin'rs, 23 N. Y 219; State v. Jones, 51 Ohio St. 492, 37 N. E. 945; Burrall v. Raliroad Co., 75 N. Y. 216.

Originally "the capital stock of the bank" was all the property of every kind, everything, which the bank possessed. And this "capital stock," nil of it, in reality belonged t6 the contributors, it being entrusted to the bank to be used and traded with for their exclusive benefit; and thus the bank became the agent of the contributors, so that the transmutation of the money originally advanced by the subscribers into property of other kinds, though it altered the form of the investment, left its beneficial ownership unaffected ; and every new acquisition of property, by exchange or otherwise, was an acquisition for the original subscribers or their representatives, their respective interests in it all always continuing in the same proportion as in the aggregate capital originally advanced. So that, whether in the form of money, bills of exchange, or any other property in possession or in action into which the money originally contributed has been changed, or which it has produced, all is, as the original contribution was, the capital stock of the bank, held, as the original contribution was, for the exclusive benefit of the original contributors and those who represent them. The original contributors and those who represent them are the stockholders. New Haven v. City Bank, 31 Conn. 109.

Capital stock, as employed in acts of incorporation, is never used to indicate the value of the property of the company. It is very generally, if not universally, used to designate the amount of capital prescribed to be contributed at the outset by the stockholders, for the purposes of the corporation. The value of the corporate assets may be greatly increased by surplus profits, or be diminished by losses, but the amount of the capital stock remains the same. The funds of the company may fluctuate ; its capital stock remains invariable, unless changed by legislative authority. Canfield v. Fire Ass'n, 23 N. J. Law, 195.
-- Black's Law Dictionary
Legal Definition
The sum subscribed by the stockholders of a corporation. See 126 N. Y. 433, 27 N. E. 818, 12 L. R. A. 762.
-- Ballentine's Law Dictionary