An index, or indicator, is a statistical measure that represents the value of a specific set of data points. It is often used to track changes over time, providing insights into trends and patterns within a particular area, such as the economy or stock market. For example, the Consumer Price Index (CPI) measures changes in the price level of a basket of consumer goods and services, helping to assess inflation. Indices can also serve as benchmarks for performance comparison. In finance, the S&P 500 index tracks the performance of 500 large companies in the U.S., allowing investors to gauge market trends and make informed decisions.