A stock index, also known as a stock market index, is a measure of the performance of a group of stocks representing a specific sector, industry, or country. It is calculated based on the weighted average of the prices of a specific selection of stocks. Stock indices are used to track the overall performance of the stock market and to provide a benchmark against which individual stocks or portfolios can be compared. Examples of widely followed stock indices include the S&P 500, the Dow Jones Industrial Average, and the NASDAQ Composite.