The number of shares in an exchange traded fund (ETF). An ETF is similar to a mutual fund, but it has the objective of achieving the same return as a particular market index. The size of the creation unit varies by fund and ranges from 25,000 to 600,000 shares. Participants engaging in the creation and redemption process are charged a fee, which varies. One well-known ETF is Spiders, or SPDRs, which invests in all the stocks included in the S&P 500 Composite Stock Price Index.