The index found in a book is a list of the topics, names and places mentioned in it, together with the page numbers where they can be found. The index is usually found at the back of a book.
Only those words that have relevance to the book's main themes are chosen for the index. An index is helpful to a reader who needs to find information quickly without having to read through the whole book. Unlike the table of contents, where the arrangement of the titles is based on the order in which they occur in the book, the topics in the index are arranged alphabetically.