pla·gia·rism
... is the action or practice of taking someone else's work, idea, etc., and passing it off as one's own; literary theft
Oxford English Dictionary, s.v. “plagiarism (n.), sense 2,” July 2023, https://www.oed.com/dictionary/plagiarism_n?tab=meaning_and_use#29927356
... to steal and pass off (the ideas or words of another) as one's own : use (another's production) without crediting the source
Merriam-Webster.com Dictionary, s.v. “plagiarism,” accessed March 22, 2024, https://www.merriam-webster.com/dictionary/plagiarism.
Self-plagiarism refers to the practice of an author using portions of their previous writings on the same topic in another of their publications, without specifically citing it formally in quotes. This practice is widespread and sometimes unintentional, as there are only so many ways to say the same thing on many occasions, particularly when writing the Methods section of an article. Although this usually violates the copyright that has been assigned to the publisher, there is no consensus as to whether this is a form of scientific misconduct, or how many of one's own words one can use before it is truly "plagiarism." Probably for this reason self-plagiarism is not regarded in the same light as plagiarism of the ideas and words of other individuals. If journals have developed a policy on this matter, it should be clearly stated for authors.
"It is important to note that the standard process of publication in many journals includes ceding copyright of the finished paper to the publisher. While you are still the intellectual owner of the ideas and results, the publication is property of the journal. As such, reuse of that material without citation and/or permission is not acceptable. While this is counterintuitive, in the eyes of the law, reusing your own words is copyright infringement, even if you wrote them."
"Open access journals commonly use Creative Commons licenses allowing for reuse with attribution. In these cases, reuse of your own words is acceptable, but it is always necessary to cite the original publication."