It stands for index-linked security; a note (debt security/ debt instrument) in which the return (coupon payments) and/ or redemption (principal) amounts are linked to the performance of either a single index or a number (basket) of indices (a stock market or price index, etc.) The value of an index-linked note reflects the cash flows of asset pools whose performance is collectively  measured by the performance of an underlying aggregated index. The note is linked to a basket of stocks or to other composite securities representing a static/ constant collection of underlying assets over a relatively long period.

Index-linked securities (ILSs), in which the underlying is an equity index, are known as equity index-linked notes or real yield securities (REALS). The underlying assets are individual equity securities constituting the equity index.

An index-linked security is also called an index-linked note.