An exchange-traded fund (or ETF) is an investment vehicle traded on stock exchanges, much like stocks or bonds. An ETF holds assets such as stocks, bonds, or futures. Institutional investors can redeem large blocks of shares of the ETF (known as "creation units") for a "basket" of the underlying assets or, alternately, exchange the underlying assets for creation units. This creation and redemption of shares enables institutions to engage in arbitrage and causes the value of the ETF to approximate the net asset value of the underlying assets. Most ETFs track an index, such as the Dow Jones Industrial Average or the S&P 500.