A citation provides information needed to locate a book, journal article or other resource. These are usually listed in a bibliography, index or database.
Book: Generally a book citation would include author, title, place of publication, publisher and date of publication.
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Journal article: Generally, a journal article citation will include author, title of the article, title of the journal in which the article appeared, volume number, issue number, date of publication and pages on which the article appeared.
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Different disciplines may use different forms or styles to cite references. These are called citation styles.