S&P 500
A market-cap-weighted index of 500 large US companies. The most widely used benchmark for US stock market performance.
The S&P 500 is maintained by S&P Dow Jones Indices and tracks 500 large-cap US-listed companies selected by a committee based on market cap, liquidity, profitability, and other factors. Its full name is the Standard & Poor's 500.
Being market-cap weighted means the largest companies — Apple, Microsoft, Nvidia — have the most influence on the index level. The top 10 holdings can represent 30%+ of the index, creating concentration risk.
The S&P 500 is the professional benchmark: fund managers are measured against it, retirement accounts default to S&P 500 index funds, and options on the index (SPX) are among the most liquid instruments in the world.