according to Merriam-Webster Dictionary index is
a list (as of bibliographical information or citations to a body of literature) arranged usually in alphabetical order of some specified datum (as author, subject, or keyword): as a list of items (as topics or names) treated in a printed work that gives for each item the page number where it may be found
convenient way for quick lookup of information in paper book (no full-text search button yet)
index terms must be marked manually
hard and time-consuming process
index can be built from index terms automatically
„just“ collect, group and sort index terms from document