US SPX 500 is a CFD product offered by OANDA, with the S&P 500 index as its underlying reference asset.

The S&P 500 is a stock market index calculated and published by S&P Dow Jones Indices, comprised of 500 companies selected from those listed on exchanges such as the New York Stock Exchange.

The index consists of highly liquid large-cap stocks and serves as a benchmark for various investment products.

Further reading: What are stock market indices? How to invest in them?