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.