Wikipedia:No original research
Wikipedia articles must not contain original research. The phrase "original research" (OR) is used on Wikipedia to refer to material—such as facts, allegations, and ideas—for which no reliable, published sources exist. This includes any analysis or synthesis of published material that serves to reach or imply a conclusion not stated by the sources. To demonstrate that you are not adding original research, you must be able to cite reliable, published sources that are directly related to the topic of the article, and directly support the material being presented. (This policy of no original research does not apply to talk pages and other pages which evaluate article content and sources, such as deletion discussions or policy noticeboards.)
|This page in a nutshell: Wikipedia does not publish original thought. All material in Wikipedia must be attributable to a reliable, published source. Articles may not contain any new analysis or synthesis of published material that serves to reach or imply a conclusion not clearly stated by the sources themselves.|
The prohibition against original research means that all material added to articles must be attributable to a reliable, published source, even if not actually attributed. The verifiability policy says that an inline citation to a reliable source must be provided for all quotations, and for anything challenged or likely to be challenged—but a source must exist even for material that is never challenged. For example: the statement "the capital of France is Paris" needs no source, nor is it original research, because it's not something you thought up and is easily verifiable; therefore, no one is likely to object to it and we know that sources exist for it even if they are not cited. The statement is attributable, even if not attributed.
Despite the need for reliable sources, you must not plagiarize them or violate their copyrights. Rewriting source material in your own words, while retaining the substance, is not considered to be original research.
"No original research" (NOR) is one of three core content policies that, along with Neutral point of view and Verifiability, determines the type and quality of material acceptable in articles. Because these policies work in harmony, they should not be interpreted in isolation from one another, and editors should familiarize themselves with all three. For questions about whether any particular edit constitutes original research, see the No original research noticeboard.