Indices is the plural form of index. An index is a list that helps you find information quickly, like the alphabetical list at the back of a textbook showing which page contains information about volcanoes or photosynthesis. Libraries use card indices (now usually digital) to organize their books. A stock market index tracks how groups of companies are performing: the Dow Jones Industrial Average is an index that follows 30 major American companies.

In mathematics, indices refers to the small numbers written above and to the right of other numbers, showing how many times to multiply that number by itself. In the expression 5³, the 3 is an index (also called an exponent or power). It means 5 × 5 × 5, which equals 125. When mathematicians work with multiple expressions like 2⁴ and 3², they're working with indices.

The singular form index can also mean your index finger, the pointer finger next to your thumb, named because people often use it to point at things in lists or books.