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Broadly, a citation is a reference to a published or unpublished source (not always the original source).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.

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