Definition of an index fund. An index mutual fund or ETF (exchange-traded fund) tracks the performance of a specific market benchmark—or "index," like the popular S&P 500 Index—as closely as possible. That's why you may hear people refer to indexing as a "passive" investment strategy. Click the tab titled “Add another Vanguard mutual fund” and enter the ticker, name, or symbol of the fund you’re looking to invest in. In the future, if you choose to invest in this fund again, it The ETF is Vanguard cheap. On an investment of $10,000, the 0.11% expense ratio works out to a mere $11 a year. Further holding down costs, the fund trades infrequently. On average, expect a stock to stay in the fund 10 years. Like conventional index mutual funds, the ETF weights stocks by their market cap—that is,