The Oxford English Dictionary (OED) Online defines plagiarism as "the action or practice of taking someone else's work, idea, etc., and passing it off as one's own; literary theft." It is plagiarism when you buy or use a term paper someone else wrote; cut and paste parts of an online book or article without citing the source; use, or even paraphrase another author's words or ideas without citing him or her. Whether it is done deliberately or accidentally, it is plagiarism.
You must give credit whenever you use:
Common knowledge is factual information considered to be in the public domain that the average reader would know such as birth and death dates of well known figures, political, literary, and other historical events.
How do you
determine if something is common knowledge? Ask yourself if you knew the information
before you began your research. If you didn't, the information is not common knowledge.
Even with this gauge, common knowledge is tricky because what is perceived as common knowledge for you may not be to someone else. You may consider information that is undocumented in multiple standard reference works as common knowledge. Additionally, it might be common knowledge if you think the information you are presenting is something your readers will already know however, the best rule is when in doubt, cite!
"The University recognizes plagiarism as a serious academic offense. Plagiarism, the act of representing the work of another as one's own, may take two forms. It may consist of copying, paraphrasing or otherwise using the written or oral work of another without acknowledging the source, or it may consist of presenting oral or written course work prepared by another as one's own."