Equity indices, or stock indices as they are also commonly known, are actual stock market indexes, which measure the value of a specific section of a stock market. They are calculated based on a weighted average of the prices of selected stocks, which belong to the actual category that they represent. Stock indices can represent a specific stock market such as NASDAQ, or they can represent a specific set of the largest companies of a nation such as the American S&P 500, the British FTSE 100, or the Japanese Nikkei 225.

The purpose of the indices is to show the general direction of a specific stock market or of the general economy of a nation. However, since stock indices are composed of a basket of companies they can be very much affected by a big move of a specific company or by a big move of a specific sector of trade.