A stock index is a weighted average of a specific collection of shares that reflects the development of (segments of) a stock market. This index provides an indication of the performance of the market or a part thereof.

Examples of stock indices are the AEX index, which includes the 25 most active stocks on NYSE Euronext, and the Midkap index for the next 25 stocks.

Stock indices are used as benchmarks to measure the performance of individual stocks or investment portfolios.