Broadly, a citation is a reference to a published or unpublished source (not always the original source).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. As Roark and Emerson have argued, citations relate to the way authors perceive the substance of their work, their position in the academic system, and the moral equivalency of their place, substance, and words.  Despite these attributes, many drawbacks and shortcoming of citation practices have been reported, including for example honorary citations, circumstantial citations, discriminatory citations, selective and arbitrary citations. 
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.
Citation content can vary depending on the type of source and may include:
- Book: author(s), book title, publisher, date of publication, and page number(s) if appropriate.
- Journal: author(s), article title, journal title, date of publication, and page number(s).
- Newspaper: author(s), article title, name of newspaper, section title and page number(s) if desired, date of publication.
- Web site: author(s), article and publication title where appropriate, as well as a URL, and a date when the site was accessed.
- Play: inline citations offer part, scene, and line numbers, the latter separated by periods: 4.452 refers to scene 4, line 452. For example, "In Eugene Onegin, Onegin rejects Tanya when she is free to be his, and only decides he wants her when she is already married" (Pushkin 4.452-53).
- Poem: spaced slashes are normally used to indicate separate lines of a poem, and parenthetical citations usually include the line number(s). For example: "For I must love because I live / And life in me is what you give." (Brennan, lines 15–16).
- Interview: name of interviewer, interview descriptor (ex. personal interview) and date of interview.
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.
- Citations of books may include an International Standard Book Number (ISBN).
- Specific volumes, articles or other identifiable parts of a periodical, may have an associated Serial Item and Contribution Identifier (SICI) or an International Standard Serial Number (ISSN).
- Electronic documents may have a digital object identifier (DOI).
- Biomedical research articles may have a PubMed Identifier ( PMID).
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 five stages of grief are denial, anger, bargaining, depression, and acceptance." 1
The note, located either at the foot of the page (footnote) or at the end of the paper (endnote) would look like this:
- 1. Elisabeth Kübler-Ross, On Death and Dying (New York: Macmillan, 1969) 45–60.
In a paper with a full bibliography, the shortened note might look like:
- 1. Kübler-Ross, On Death and Dying 45–60.
The bibliography entry, which is required with a shortened note, would look like this:
- Kübler-Ross, Elisabeth. On Death and Dying . New York: Macmillan, 1969.
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.
An example of a parenthetical reference:
- "The five stages of grief are denial, anger, bargaining, depression, and acceptance" (Kübler-Ross,1969, p 45–60).
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 Chicago Style (CMOS) was developed and its guide is The Chicago Manual of Style . It is most widely used in history and economics as well as some social sciences. The closely related Turabian style—which derives from it—is for student references, and is distinguished from the CMOS by omission of quotation marks in reference lists, and mandatory access date citation.
- The Columbia Style was created by Janice R. Walker and Todd Taylor to give detailed guidelines for citing internet sources. Columbia Style offers models for both the humanities and the sciences.
- Evidence Explained: Citing History Sources from Artifacts to Cyberspace by Elizabeth Shown Mills covers primary sources not included in CMOS, such as censuses, court, land, government, business, and church records. Includes sources in electronic format. Used by genealogists and historians.
- Harvard referencing (or author-date system) is a specific kind of parenthetical referencing. Parenthetical referencing is recommended by both the British Standards Institution and the Modern Language Association. Harvard referencing involves a short author-date reference, e.g., "(Smith, 2000)", being inserted after the cited text within parentheses and the full reference to the source being listed at the end of the article.
- MLA style was developed by the Modern Language Association and is most often used in the arts and the humanities, particularly in English studies, other literary studies, including comparative literature and literary criticism in languages other than English (" foreign languages "), and some interdisciplinary studies, such as cultural studies, drama and theatre, film, and other media, including television. This style of citations and bibliographical format uses parenthetical referencing with author-page (Smith 395) or author-[short] title-page (Smith, Contingencies 42) in the case of more than one work by the same author within parentheses in the text, keyed to an alphabetical list of sources on a "Works Cited" page at the end of the paper, as well as notes (footnotes or endnotes). See The MLA Style Manual and The MLA Handbook for Writers of Research Papers , particularly Citation and bibliography format.
- The MHRA Style Guide is published by the Modern Humanities Research Association (MHRA) and most widely used in the arts and humanities in the United Kingdom, where the MHRA is based. It is available for sale both in the UK and in the United States. It is similar to MLA style, but has some differences. For example, MHRA style uses footnotes that reference a citation fully while also providing a bibliography. Some readers find it advantageous that the footnotes provide full citations, instead of shortened references, so that they do not need to consult the bibliography while reading for the rest of the publication details.
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.
- The Bluebook is a citation system traditionally used in American academic legal writing, and the Bluebook (or similar systems derived from it) are used by many courts. At present, academic legal articles are always footnoted, but motions submitted to courts and court opinions traditionally use inline citations, which are either separate sentences or separate clauses. Inline citations allow readers to quickly determine the strength of a source based on, for example, the court a case was decided in and the year it was decided.
- The legal citation style used almost universally in Canada is based on the Canadian Guide to Uniform Legal Citation (aka McGill Guide ), published by McGill Law Journal . 
- British legal citation almost universally follows the Oxford Standard for Citation of Legal Authorities (OSCOLA).
Sciences, mathematics, engineering, physiology, and medicine
- The American Chemical Society style, or ACS style, is often used in Chemistry and some of the physical sciences. In ACS style references are numbered in the text and in the reference list, and numbers are repeated throughout the text as needed.
- In the style of the American Institute of Physics (AIP style), references are also numbered in the text and in the reference list, with numbers repeated throughout the text as needed.
- Styles developed for the American Mathematical Society (AMS), or AMS styles, such as AMS-LaTeX, are typically implemented using the BibTeX tool in the LaTeX typesetting environment. Brackets with author's initials and year are inserted in the text and at the beginning of the reference. Typical citations are listed in-line with alphabetic-label format, e.g. [AB90]. This type of style is also called an " Authorship trigraph. "
- The Vancouver system, recommended by the Council of Science Editors (CSE), is used in medical and scientific papers and research.
- In one major variant, that used by the American Society of Mechanical Engineers (ASME), citation numbers are included in the text in square brackets rather than as superscripts. All bibliographical information is exclusively included in the list of references at the end of the document, next to the respective citation number.
- The International Committee of Medical Journal Editors (ICMJE) is reportedly the original kernel of this biomedical style, which evolved from the Vancouver 1978 editors' meeting. The MEDLINE / PubMed database uses this citation style and the National Library of Medicine provides "ICMJE Uniform Requirements for Manuscripts Submitted to Biomedical Journals -- Sample References". 
- The AMA Style.
- The style of the Institute of Electrical and Electronics Engineers (IEEE), or IEEE style, encloses citation numbers within square brackets and numbers them consecutively, with numbers repeated throughout the text as needed. 
- Pechenik Citation Style is a style described in A Short Guide to Writing about Biology , 6th ed. (2007), by Jan A. Pechenik. 
- In 1955, Eugene Garfield proposed a bibliographic system for scientific literature, to consolidate the integrity of scientific publications. 
- The style of the American Psychological Association, or APA style, published in the Publication Manual of the American Psychological Association, is most often used in social sciences. APA citation style is similar to Harvard referencing, listing the author's name and year of publication, although these can take two forms: name citations in which the surnames of the authors appear in the text and the year of publication then appears in parentheses, and author-date citations , in which the surnames of the authors and the year of publication all appear in parentheses. In both cases, in-text citations point to an alphabetical list of sources at the end of the paper in a References section.
- The American Political Science Association publishes both a style manual and a style guide for publications in this field.  The style is close to the CMOS.
- The American Anthropological Association utilizes a modified form of the Chicago Style laid out in their Publishing Style Guide. 
- The ASA style of American Sociological Association is one of the main styles used in sociological publications.
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 instance 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. Whilst in Sociology the number of references, the article length, and title length are among the factors. 
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 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. 
- The field of Communication (or Communications) overlaps with some of the disciplines also covered by the MLA and has its own disciplinary style recommendations for documentation format; the style guide recommended for use in student papers in such departments in American colleges and universities is often The Publication Manual of the APA ( American Psychological Association); designated for short as " APA style ".