S&P 500

The S&P 500 tracks roughly 500 large US companies weighted by market value, and it is the default meaning of 'the market' in American finance. Membership is decided by a committee against liquidity and profitability rules, not purely by size. Its concentration is often underappreciated: the largest ten holdings have at times exceeded a third of the index, so an S&P 500 fund is diversified across sectors but heavily exposed to a few very large firms.