Fixed income refers to any type of investment under which the borrower or issuer is obliged to make payments of a fixed amount on a fixed schedule. For example, the borrower may have to pay interest at a fixed rate once a year and repay the principal amount on maturity. Fixed-income securities — more commonly known as bonds — can be contrasted with equity securities – often referred to as stocks and shares – that create no obligation to pay dividends or any other form of income. Bonds carry a level of legal protections for investors that equity securities do not — in the event of a bankruptcy, bond holders would be repaid after liquidation of assets, whereas shareholders with stock often receive nothing.

For a company to grow its business, it often must raise money – for example, to finance an acquisition; buy equipment or land, or invest in new product development. The terms on which investors will finance the company will depend on the risk profile of the company. The company can give up equity by issuing stock or can promise to pay regular interest and repay the principal on the loan (bonds or bank loans). Fixed-income securities also trade differently than equities. Whereas equities, such as common stock, trade on exchanges or other established trading venues, many fixed-income securities trade over-the-counter on a principal basis.

The term "fixed" in "fixed income" refers to both the schedule of obligatory payments and the amount. "Fixed income securities" can be distinguished from inflation-indexed bonds, variable-interest rate notes, and the like. If an issuer misses a payment on fixed income security, the issuer is in default, and depending on the relevant law and the structure of the security, the payees may be able to force the issuer into bankruptcy. In contrast, if a company misses a quarterly dividend to stock (non-fixed-income) shareholders, there is no violation of any payment covenant and no default.

The term "fixed income" is also applied to a person's income that does not vary materially over time. This can include income derived from fixed-income investments such as bonds and preferred stocks or pensions that guarantee a fixed income. When pensioners or retirees are dependent on their pension as their dominant source of income, the term "fixed income" can also imply that they have relatively limited discretionary income or have little financial freedom to make large or discretionary expenditures.