An equity index is a measure of the performance of an overall stock market or a specific segment of it. It's typically calculated using a basket of securities that represent a particular market or sector, and its performance is tracked over time to gauge investor sentiment and market trends. Some common examples include the S&P 500 index in the US, which tracks the performance of 500 large-cap companies, and the NASDAQ Composite, which measures the stocks listed on the NASDAQ exchange. Equity indices are often used as benchmarks for investment portfolios or to gauge the overall health of an economy.