Refers to any type of investment that yields a regular (or fixed) return.
For example, if you lend money to a borrower and the borrower has to pay interest once a month, you have been issued a fixed-income security. Governments issue government bonds in their own currency and sovereign bonds in foreign currencies. Local governments issue municipal bonds to finance themselves. Debt issued by government-backed agencies is called an agency bond. Companies can issue a corporate bond or get money from a bank through a corporate loan ("preferred stock" is also sometimes considered to be fixed income). Secularized bank lending (e.g. credit card debt, car loans or mortgages) can be structured into other types of fixed income products such as ABS - asset-backed securities which can be traded on exchanges just like corporate and government bonds.