A certificate of deposit whose return is based on changes in an underlying index, common benchmark or even the fluctuation of a foreign currency. Some are indexed to the S&P 500, or the prime rate, or Series EE savings bonds or the yield on Treasury bills. Others are linked to foreign currencies or stock exchanges. Returns on these CDs are not guaranteed. If the index falls during the time you hold the CD, you earn nothing. But the principal is insured by the FDIC, just like a fixed-rate CD.