A stock index, or stock market index, is an index that measures a stock market, or a subset of the stock market, that helps investors compare current price levels with past prices to calculate market performance. It is computed from the prices of selected stocks (typically a weighted arithmetic mean).
Two of the primary criteria of an index are that it is investable and transparent: The methods of its construction are specified. Investors can invest in a stock market index by buying an index fund, which are structured as either a mutual fund or an exchange-traded fund, and "track" an index. The difference between an index fund's performance and the index, if any, is called tracking error. For a list of major stock market indices, see List of stock market indices.