A bond index is a measure of the performance of a group or basket of bonds. It is typically composed of a fixed income securities, such as government or corporate bonds, and represents an average or benchmark of how these bonds are performing in the market. Bond indices can be used to track the overall performance of the bond market, measure the return on specific types of bonds, or as a tool for investors when making decisions about which bonds to purchase. There are many different types of bond indices, such as the Barclays Capital U.S. Government/Credit Bond Index, the Lehman Brothers U.S. Treasury Bond Index, and the Merrill Lynch High Yield Master II Index, each with their own specific composition and methodology for calculating returns.