An index (plural: usually indexes, more rarely indices; see below) 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 collection of documents. Examples are an index in the back matter of a book and an index that serves as a library catalog.
In a traditional back-of-the-book index, the headings will include names of people, places, events, and concepts selected by the indexer as being relevant and of interest to a possible reader of the book. The indexer may be the author, the editor, or a professional indexer working as a third party. 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.