The index that tracks the 500 largest US publicly traded companies is the S&P 500.
The name stands for Standard & Poor’s 500. It was introduced on March 4, 1957, and is maintained by S&P Dow Jones Indices. Unlike a simple list ranked only by company size, it uses a committee and eligibility rules covering factors such as market capitalization, liquidity, public float, profitability, and US domicile.
The index is weighted by float-adjusted market capitalization, meaning larger companies have a greater effect on its value. Its constituents collectively represent roughly 80% of the total market value of publicly traded US companies, which is why it is often treated as a broad snapshot of the US stock market.
A common mix-up is assuming that the S&P 500 contains exactly 500 stocks at every moment. Companies can have multiple share classes, so the index may contain more than 500 securities while still representing about 500 major companies. The Russell 2000 instead focuses on smaller companies, while the Dow Jones Industrial Average contains only 30 large companies.