Also known as Purchasing Power Risk, this risk arises from the decline in value of securities cash flow due to inflation, which is measured in terms of purchasing power.
Here's how it works:
You buy a bond with a coupon rate of 4%. The inflation rate is at 2%. Even though you are earning 4% on your money, inflation is chipping 2% of it away only leaving you with 2% of your money or purchase power, which you can use when you receive your payments.
Only Inflation Protection Bonds such as TIPS offer protection against this risk. Floaters help reduce this risk because of the resetting of the interest rates. All other bonds expose the investor to this risk because the interest rate is fixed for the life of the bond.