I is for Index

An index is a list of important topics, usually found at the back of a book. It gives the location where each item appears, helping you find information quickly. An index is often arranged in alphabetical order and may include smaller topics under a main topic. It is different from a table of contents, which lists chapters in the order they appear. The word comes from a Latin word meaning "pointer" or "sign."

An index can also be a number or sign that shows the size or change of something, such as a price index.

The picture is of an old card index, where you can look up where things are in a library or collection of things.