INX stands for “S&P 500 Index”. It is a stock market index that measures the stock performance of 500 large companies listed on stock exchanges in the United States. It is one of the most widely-followed equity indices and is used as a benchmark for the overall performance of the US stock market. The index is maintained by S&P Dow Jones Indices, a division of S&P Global, and its value is calculated using a market capitalization-weighted methodology.