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. An index differs from a word index, or concordance, in focusing on the subject of the text rather than the exact words in a text, and it differs from a table of contents because the index is ordered by subject, regardless of whether it is early or late in the book, while the listed items in a table of contents is placed in the same order as the book.

In a traditional back-of-the-book index, the headings will include names of people, places, events, and concepts selected as being relevant and of interest to a possible reader of the book. The indexer performing the selection 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.

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. The concept embodied by book indexes lent its name to database indexes, which similarly provide an abridged way to look up information in a larger collection, albeit one for computer use rather than human use.