A stock index is a statistical measure that tracks the performance of a group of stocks representing a particular market, sector, or investment theme. Major indices include the S&P 500, Dow Jones, NASDAQ, and international benchmarks that provide reference points for investment performance and market analysis.

Stock indices use different weighting methods including market capitalization, price weighting, or equal weighting, affecting how individual stocks influence index performance. Index futures and options enable efficient hedging and speculation on broad market movements. Understanding index construction helps assess market representation and trading opportunities.

Real-world example: The NASDAQ Composite index gains 15% driven by technology stock outperformance, prompting increased investment flows into technology-focused ETFs and encouraging sector rotation strategies favoring growth stocks.