A concordance is a book that lists every word that appears in another book alphabetically, alongside all the places in which that word appears. The most common concordances are biblical concordances, which are used by scholars and lay people to find what the Bible has to say on any given topic. For example, looking up 'leviathan' reveals that the beast is referenced twice in Job, twice in the Psalms and once in Isaiah, usually when making reference to the great power of God.
Other books that commonly have concordances include the works of Shakespeare and Dante.
With the advent of the information age, search technology has outstripped the power of paper concordances, and the most efficient method of finding a particular word is usually to use a digital search engine. Nevertheless, a traditional concordance can be a useful and convenient addition to any home library.