The word “index” means many things, most of which relate to pointing in some way (“index”, plural “indicis”, is Latin for forefinger, informer, or sign). The sense we are interested in is that given in the Concise Oxford Dictionary as: 6. Alphabetical list, usu. at end of book, of names, subjects, etc., with references; A traditional index is in fact a map of the knowledge contained in a book; it lists the topics covered, by whatever name users might be expected to want to look them up, and includes salient (and only salient) references to those topics.