A financial instrument represents the ownership of cash stream. A piece of paper rather than a
thing. A promise.
Fixed income securities are financial instruments that trade in well-developed markets and give
you ownership of a definite promised cash flow stream.
“Definite” used to mean fixed, specified. Now it’s more general. It might mean contingent on
some other economic variable (LIBOR) or some other security’s value, and so could be
contingent, but as long as it generates a well-defined cash flow stream it’s generally considered
a fixed income security. Not a precise definition.