• A manually created, systematic arrangement of key words and modifying phrases designed to enable users to locate information in a publication. • The process of creating “back of the book” indexes is not a mechanical one. It is a form of writing and like other types of writing; it is a mixture of analysis, judgment, and selection. • Indexing is not just a matter of picking out names or ‘significant’ words, and index entries are made only if they lead to worthwhile information. The final index entries can only be decided in the light of the content and context of the book as a whole.
An index is much more than an alphabetical list of topics with page numbers attached – at least, it should be. A good index is a road map that leads both experts and novices in your field to every pertinent sentence you’ve written, regardless of their point of departure and without dead ends or annoying detours. —American Society of Indexing