Self-plagiarism seems like a self-canceling term. After all, plagiarism is normally defined as the uncredited use of someone else’s words or ideas (see Purdue OWL). How, then, could people possibly plagiarize themselves?
The idea of self-plagiarism may come up in situations where writers have given the rights to their work to someone else. For example, at the content-writing site Demand Studios, writers are told that “self-plagiarism is not allowed.” When writers sell an article to the company, the company acquires all the copyrights to the work. In other words, the company becomes the sole owner of the article.
Say a writer at DS is working on a new article that’s on a topic similar to an article he wrote and sold to the company earlier in the year. If the writer copies-and-pastes entire sections of his old article into his new article, he is actually violating the copyright of the company, because the company, not the writer, now owns all the copyrights to the old article.
Strictly speaking, by the conventional definition of plagiarism, the writer is not actually plagiarizing his old work, because the ideas and words he is using were originally his. But many people accept the term now, and use it to refer to somebody copying portions of his work from articles she has already sold to the company.
Less commonly, the term “self-plagiarism” may be used in situations where a company does not claim all rights in articles, but does want all work submitted to be unique. For example, when writers publish articles on Helium, they give Helium a license to use the articles, but they retain the copyrights themselves. In this situation, the term “self-plagiarism” might be used to refer to a writer who publishes duplicate or near-duplicate articles under different titles on Helium. This is against the company’s rules, as the company does not want to pay twice for the same material, but it is again not strictly true to call it “plagiarism,” at least not by the traditional definition.
According to Wikipedia, the idea of self-plagiarism is also used in academic settings, where it is applied to such things as students submitting the same paper in two different courses, researchers using the results of a single study to generate multiple journal articles, and academics publishing the same paper in multiple publications without acknowledging the duplication.
So while there is controversy over whether the term “self-plagiarism” actually accurately describes anything, the term is being used in schools and in publishing to describe the actions of authors who reuse their own work in situations where, for various possible reasons, that kind of reuse is not allowed.