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An index is a list of words and associated pointers to where those words can be found in a document. In a traditional back-of-the-book index the words (or phrases) are concepts selected by a person and the pointers are page 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, are indexes too. The words are words found in HTML (or other documents), and the pointers are URL's.

Indexes are designed to help the reader find information quickly and easily. A complete and truly useful index is not simply a list of the words and phrases used in a publication (which is properly called a concordance), but an organized map of its contents, including cross-references, grouping of like concepts, and other useful intellectual analysis.

Sample back-of-the-book index excerpt:

sage, 41-42. See also Herbs ← directing the reader to related terms

Scarlet Sages. See Salvia coccinea ← redirecting the reader to term used in the text

shade plants ← grouping term (may not appear in the text; may be generated by indexer)

hosta, 93 ← subentries

myrtle, 46

Solomon's seal, 14

sunflower, 47 ← regular entry

In books, indexes are usually placed near the end (this is commonly known as "BoB" or back-of-book indexing). They complement the table of contents by enabling access to information by specific subject, whereas contents listings enable access through broad divisions of the text arranged in the order they occur.

The indexing process usually begins with a reading of the text, during which the terms to be used are selected and sometimes marked (e.g. with a highlighter). The indexer then makes a second pass through the text during which he or she enters the terms into an index document, creating subentries where appropriate. The final task involves arranging the index document into alphabetical order and going through it grouping like terms, adding cross-references where appropriate, and editing to improve consistency, accuracy, and usefulness, and to ensure it follows publisher's guidelines.

Indexers must analyze the text to enable presentation of concepts and ideas in the index that may not be named within the text. The index is meant to help the reader, researcher, or information professional, not the author, find information, so the professional indexer must act as a liaison between the text and the its ultimate user.

Indexing is often done by freelancers hired by publishers or book packagers. Some publishers and database companies employ indexers.

There are several dedicated, indexing software programs available to assist with the special sorting and copying needs involved in index preparation. The most widely known include Cindex, Macrex, and SkyIndex.

Increasing interest in the use of electronic documents has led to the development of embedded indexing, where index terms are inserted into appropriate places in one or more source documents using some kind of markup language. An accurate, sorted list of these marked index terms can then be generated dynamically from the source document(s) at any time. This is a standard, yet little known, feature of many popular word processing programs such as Microsoft Word, StarWriter, and WordPerfect. The lists generated are more like the concordances discussed above; they are not indexes.

Some indexers specialize in specific formats such as scholarly books, microforms, web indexing (the application of a back-of-book-style index to a website or intranet), search engine indexing, database indexing (the application of a pre-defined controlled vocabulary such as MeSH to articles for inclusion in a database), periodical indexing (indexing of newspapers, journals, magazines).

With their expertise in controlled vocabularies, some indexers also works as taxonomists and ontologists.

Some indexers specialize in particular subject areas, such as anthropology, business, computers, economics, education, government documents, history, law, mathematics, medicine, psychology, and technology. An indexer can be found for any subject.

• Booth, Pat 2001, Indexing: the manual of good practice (K.G. Saur), ISBN 3-598-11536-9
• Browne, Glenda and Jon Jermey 2007, The Indexing Companion (Cambridge University Press), ISBN 978-0-52168-988-5
• Mulvany, Nancy 2005, Indexing Books, Second Edition (University of Chicago Press) ISBN 0-226-55276-4
• Smith, Sherry and Kari Kells 2005, "Inside Indexing: the Decision-Making Process" (Northwest Indexing Press), ISBN 0-9771035-01
• Stauber, Do Mi 2004, "Facing the Text: Content and Structure in Book Indexing" (Cedar Row Press) ISBN 0-9748345-0-5
• Wellisch, Hans 1995, "Indexing from A to Z", Second Edition (HW Wilson) ISBN 0-8242-0807-2

• ISO 999:1996 Guidelines for the Content, Organization, and Presentation of Indexes (this is also the national standard in the UK, Australia, and New Zealand)

• Indexing Society of Canada
• American Society of Indexers
• Australian and New Zealand Society of Indexers
• British Society of Indexers
• China Society of Indexers

• The Indexer (the international journal)
• Author-Created Indexes
• Consideration in Indexing Online Documents
• Complexity In Indexing Systems
• Reflections on Authorship and Indexing
• Usability studies for indexes
• "The Definite Article: Acknowledging 'The' in Index Entries", Glenda Browne, The Indexer, vol. 22, no. 3 April 2001, pp. 119-22.