Citation indexes are unique from the point of view of technique followed. They are based on citations made in current documents. They follow a novel method of detecting subject relationship of documents through citations, and are claimed to be a useful tool for subject retrieval. The fact of the association from citation is transformed into an index keeping the direction from cited to citing documents. The ALA Glossary of Library and Information Science (1983) defines Citation Index as an index consisting essentially of a list of works which have been cited in another, later works and a list of the works from which the citations have been collected.
Citation Index is the contribution of Eugene Garfield of the "Institute for Scientific Information"(ISI); Philadelphia, USA.