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

Examples of stock indices include the AEX index, which comprises the 25 most active stocks on NYSE Euronext, and the Midkap index for the 25 stocks following it.

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