What is a stock index?

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 offers 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 shares on NYSE Euronext, and the Midkap-index for the 25 subsequent shares.

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