A citation is a reference to a source. More precisely, a citation is an abbreviated alphanumeric expression embedded in the body of an intellectual work that denotes an entry in the bibliographic references section of the work for the purpose of acknowledging the relevance of the works of others to the topic of discussion at the spot where the citation appears.
Generally the combination of both the in-body citation and the bibliographic entry constitutes what is commonly thought of as a citation (whereas bibliographic entries by themselves are not). References to single, machine-readable assertions in electronic scientific articles are known as nanopublications, a form of microattribution.
Citations have several important purposes: to uphold intellectual honesty (or avoiding plagiarism), to attribute prior or unoriginal work and ideas to the correct sources, to allow the reader to determine independently whether the referenced material supports the author's argument in the claimed way, and to help the reader gauge the strength and validity of the material the author has used.
The forms of citations generally subscribe to one of the generally accepted citations systems, such as the Oxford, Harvard, MLA, American Sociological Association (ASA), American Psychological Association (APA), and other citations systems, because their syntactic conventions are widely known and easily interpreted by readers. Each of these citation systems has its advantages and disadvantages. Editors often specify the citation system to use.
Bibliographies, and other list-like compilations of references, are generally not considered citations because they do not fulfill the true spirit of the term: deliberate acknowledgement by other authors of the priority of one's ideas.
A bibliographic citation is a reference to a book, article, web page, or other published item. Citations should supply detail to identify the item uniquely. Different citation systems and styles are used in scientific citation, legal citation, prior art, the arts, and the humanities. Regarding the use of citations in the scientific literature, some scholars also put forward "the right to refuse unwanted citations" in certain situations deemed inappropriate.
Citation content can vary depending on the type of source and may include:
Along with information such as author(s), date of publication, title and page numbers, citations may also include unique identifiers depending on the type of work being referred to.
Broadly speaking, there are two types of citation systems, the Vancouver system and parenthetical referencing. However, the Council of Science Editors (CSE) adds a third, the citation-name system.
The Vancouver system uses sequential numbers in the text, either bracketed or superscript or both. The numbers refer to either footnotes (notes at the end of the page) or endnotes (notes on a page at the end of the paper) that provide source detail. The notes system may or may not require a full bibliography, depending on whether the writer has used a full-note form or a shortened-note form.
For example, an excerpt from the text of a paper using a notes system without a full bibliography could look like:
The note, located either at the foot of the page (footnote) or at the end of the paper (endnote) would look like this:
In a paper with a full bibliography, the shortened note might look like:
The bibliography entry, which is required with a shortened note, would look like this:
In the humanities, many authors also use footnotes or endnotes to supply anecdotal information. In this way, what looks like a citation is actually supplementary material, or suggestions for further reading.
Parenthetical referencing, also known as Harvard referencing, has full or partial, in-text, citations enclosed in circular brackets and embedded in the paragraph.
Depending on the choice of style, fully cited parenthetical references may require no end section. Other styles include a list of the citations, with complete bibliographical references, in an end section, sorted alphabetically by author. This section is often called "References", "Bibliography", "Works cited" or "Works consulted".
In-text references for online publications may differ from conventional parenthetical referencing. A full reference can be hidden, only displayed when wanted by the reader, in the form of a tooltip. This style makes citing easier and improves the reader's experience.
Superscripted numbers are inserted at the point of reference, just as in the citation‐sequence system, but the citations are numbered according to the order of cited works at the end of the paper or book; this list is often sorted alphabetically by author.
Citation styles can be broadly divided into styles common to the Humanities and the Sciences, though there is considerable overlap. Some style guides, such as the Chicago Manual of Style, are quite flexible and cover both parenthetical and note citation systems. Others, such as MLA and APA styles, specify formats within the context of a single citation system. These may be referred to as citation formats as well as citation styles. The various guides thus specify order of appearance, for example, of publication date, title, and page numbers following the author name, in addition to conventions of punctuation, use of italics, emphasis, parenthesis, quotation marks, etc., particular to their style.
A number of organizations have created styles to fit their needs; consequently, a number of different guides exist. Individual publishers often have their own in-house variations as well, and some works are so long-established as to have their own citation methods too: Stephanus pagination for Plato; Bekker numbers for Aristotle; citing the Bible by book, chapter and verse; or Shakespeare notation by play.
The Citation Style Language (CSL) is an open XML-based language to describe the formatting of citations and bibliographies.
In some areas of the Humanities, footnotes are used exclusively for references, and their use for conventional footnotes (explanations or examples) is avoided. In these areas, the term "footnote" is actually used as a synonym for "reference", and care must be taken by editors and typesetters to ensure that they understand how the term is being used by their authors.
In their research on footnotes in scholarly journals in the field of communication, Michael Bugeja and Daniela V. Dimitrova have found that citations to online sources have a rate of decay (as cited pages are taken down), which they call a "half-life", that renders footnotes in those journals less useful for scholarship over time.
Other experts have found that published replications do not have as many citations as original publications.
Another important issue is citation errors, which often occur due to carelessness on either the researcher or journal editor's part in the publication procedure. Experts have found that simple precautions, such as consulting the author of a cited source about proper citations, reduce the likelihood of citation errors and thus increase the quality of research.
Research suggests the impact of an article can be, partly, explained by superficial factors and not only by the scientific merits of an article. Field-dependent factors are usually listed as an issue to be tackled not only when comparison across disciplines are made, but also when different fields of research of one discipline are being compared. For example, in medicine, among other factors, the number of authors, the number of references, the article length, and the presence of a colon in the title influence the impact; while in sociology the number of references, the article length, and title length are among the factors.
Nature Index recognizes that citations remain a controversial and yet important metric for academics. They report five ways to increase citation counts: (1) watch the title length and punctuation; (2) release the results early as preprints; (3) avoid referring to a country in the title, abstract, or keywords; (4) link the article to supporting data in a repository; and (5) avoid hyphens in the titles of research articles.
Citation patterns are also known to be affected by unethical behavior of both the authors and journal staff. Such behavior is called impact factor boosting, and was reported to involve even the top-tier journals. Specifically the high-ranking journals of medical science, including The Lancet, JAMA and The New England Journal of Medicine, are thought to be associated with such behavior, with up to 30% of citations to these journals being generated by commissioned opinion articles. On the other hand, the phenomenon of citation cartels is rising. Citation cartels are defined as groups of authors that cite each other disproportionately more than they do other groups of authors who work on the same subject.