INDEXING IS A CRUCIAL and widespread activity, but it is inconspicuous. Documents need indexes to help people find specific information within them, and collections need indexes to help people locate specific items they contain. Although most people use indexes regularly, hardly anyone thinks of the creative activity that went into their making. Despite inroads being made by computers, most indexes are still created by humans.

An index is a systematic guide that helps people find information in a document, such as a book, or documents in a collection, such as records in an archive. In addition to the terms that represent the topics of the document or collection item, an index also needs a syntax that allows expression of complex topics, such as a heading with subheadings; cross-references to lead from terms to other potentially useful terms; locators, such as page numbers, or links to lead users to information about the terms they select; and a way of filing the headings or making them searchable.

A glossary is not an index, because it does not link from its entries to other content. A concordance – an alphabetised list of words in a document – is not a true index because it simply lists words and phrases from the text, without analysis. Much the same is true of search engine ‘indexes’, which rely on the actual words in a document.