In finance, a bond is a type of security under which the issuer (debtor) owes the holder (creditor) a debt, and is obliged – depending on the terms – to provide cash flow to the creditor (e.g. repay the principal (i.e. amount borrowed) of the bond at the maturity date as well as interest (called the coupon) over a specified amount of time). The timing and the amount of cash flow provided varies, depending on the economic value that is emphasized upon, thus giving rise to different types of bonds. The interest is usually payable at fixed intervals: semiannual, annual, and less often at other periods. Thus, a bond is a form of loan or IOU. Bonds provide the borrower with external funds to finance long-term investments or, in the case of government bonds, to finance current expenditure.

Bonds and stocks are both securities, but the major difference between the two is that (capital) stockholders have an equity stake in a company (i.e. they are owners), whereas bondholders have a creditor stake in a company (i.e. they are lenders). As creditors, bondholders have priority over stockholders. This means they will be repaid in advance of stockholders, but will rank behind secured creditors, in the event of bankruptcy. Another difference is that bonds usually have a defined term, or maturity, after which the bond is redeemed, whereas stocks typically remain outstanding indefinitely. An exception is an irredeemable bond, which is a perpetuity, that is, a bond with no maturity. Certificates of deposit (CDs) or short-term commercial paper are classified as money market instruments and not bonds: the main difference is the length of the term of the instrument.

The most common forms include municipal, corporate, and government bonds. Very often the bond is negotiable, that is, the ownership of the instrument can be transferred in the secondary market. This means that once the transfer agents at the bank medallion-stamp the bond, it is highly liquid on the secondary market. The price of a bond in the secondary market may differ substantially from the principal due to various factors in bond valuation.