A stock index or stock market index is a measurement of the value of a section of the stock market. It is computed from the prices of selected stocks (typically a weighted average). It is a tool used by investors and financial managers to describe the market, and to compare the return on specific investments.
An index is a mathematical construct, so it may not be invested in directly. But many mutual funds and exchange-traded funds attempt to “track” an index (see index fund), and those funds that do not may be judged against those that do.
An exchange-traded fund (ETF) is an investment fund traded on stock exchanges, much like stocks. An ETF holds assets such as stocks, commodities, or bonds and generally operates with an arbitrage mechanism designed to keep it trading close to its net asset value, although deviations can occasionally occur. Most ETFs track an index, such as a stock index or bond index. ETFs may be attractive as investments because of their low costs, tax efficiency, and stock-like features. By 2013, ETFs had become the most popular type of exchange-traded product.
A market index is an aggregate value produced by combining several stocks or other investment vehicles together and expressing their total values against a base value from a specific date. Market indexes are intended to represent an entire stock market and thus track the market’s changes over time.