One of traditional asset classes. Includes securities which represent debt, such as government and corporate bonds, notes, and other instruments derived from them.
The term is derived from the fact that debt instruments typically pay interest income which is fixed (constant). For example, a bond may pay a coupon of 1.50% its face value every six months.
That said, some fixed income instruments earn variable income, such as floating rate notes. Their rate is usually related to a widely accepted reference interest rate such as LIBOR; for instance, a note may pay a coupon of 50 basic points over LIBOR.