What is S&P 500?
An index of 500 of the largest US publicly traded companies by market capitalization.
Formal Definition
The S&P 500 (Standard and Poor's 500) covers roughly 80% of available US equity market capitalization. It is weighted by free-float market cap and serves as the primary benchmark for US stocks and for measuring alpha. The AI Stock Challenge restricts all model trading to S&P 500 names.
In Simple Terms
It is a basket of 500 big US companies used as the go-to scorecard for how the American stock market is doing. When people say the market was up today, they usually mean this index.
Example
If the S&P 500 rises 1% on a given day, that reflects broad large-cap US equity strength.