Plagiarism Plagiarism is the act of using someone else's words or ideas as if they were one's own. It is also plagiarism to change a few words and give no credit to the author. Plagiarism is a serious offense. If an original idea of an author is used, even if it is not copied word for word, that idea must be documented.
Plagiarism is essentially theft - the stealing of someone else's intellectual property.
Of course, common sense as well as ethics should determine what you document. But you must indicate the source of any appropriated material that readers might otherwise mistake for your own. If you have any doubt about whether or not you are committing plagiarism, cite your source or sources. This can extend to ideas as well as written words. Publication Manual of the American Psychological Association 4th Ed.