An index is an alphabetical list at the back of a book that helps you find specific topics quickly. Instead of flipping through hundreds of pages looking for information about volcanoes or Abraham Lincoln, you check the index, which tells you exactly which pages mention those subjects. Libraries also use indexes to help people locate articles and other resources.

In mathematics, an index is a small number written slightly above and to the right of another number, showing how many times to multiply that number by itself. In the expression 5³, the 3 is the index (also called an exponent), telling you to multiply 5 × 5 × 5.

The word also describes a measurement that tracks changes over time. The consumer price index measures how prices change for everyday goods, helping economists understand whether things are getting more or less expensive. A stock market index tracks how groups of companies are performing financially.

When something is indexed, it means it's been organized for easy searching. Your brain constantly indexes memories, filing away experiences so you can recall them later. Search engines index billions of web pages, creating enormous catalogs that let you find information in seconds.