After referring to them since the beginning of the semester, finally we define and discuss the major stock market averages and indexes that we use to measure the performance of various parts of the stock markets. As we saw in chapter 4 on mutual funds, market indexes have become very important because of the increasing number of index funds and ETFs (exchange traded funds), many of which make their way into employer sponsored retirement plans such as 401(k) and 403(b) plans.