Exchange Traded Fund.
An investment vehicle pioneered by the AMEX, these are structured equities that can be traded like stocks, and are comprised of ownership in many different stocks to reflect the make-up of an index. Not unlike an index-tracking-mutual fund except that it can be traded intra-day. The first of these products was the SPDR, or spider, SPY, which replicated the S&P 500. The most successful is the QQQ, while represents the NASDAQ 100.