An index is a list of words or phrases ('headings') and associated pointers ('locators') to where useful material relating to that heading can be found in a document or on a page. In a traditional back-of-the-book index the headings will include names of people, places and events, and concepts selected by a person as being relevant and of interest to a possible reader of the book. The pointers are typically page numbers, paragraph numbers or section numbers. In a library catalog the words are authors, titles, subject headings, etc., and the pointers are call numbers. Internet search engines, such as Google, and full text searching help provide access to information but are not as selective as an index, as they provide non-relevant links, and may miss relevant information if it is not phrased in exactly the way they expect.
Perhaps the most advanced investigation of problems related to book indexes is made in the development of topic maps, which started as a way of representing the knowledge structures inherent in traditional back-of-the-book indexes.