Abbreviation for Exchange Traded Fund, which is a fund that tracks an index, but can be traded like a stock. ETFs always bundle together the securities that are in an index; they never track actively managed mutual fund portfolios (because most actively managed funds only disclose their holdings a few times a year, so the ETF would not know when to adjust its holdings most of the time). Investors can do just about anything with an ETF that they can do with a normal stock, such as short selling.