Is plagiarism illegal?
No. Plagiarism is neither a criminal nor civil offence. In fact, plagiarism is not a legal term and is not legally recognised. But breach of copyright or intellectual property rights (IPR) is illegal; if an act of plagiarism breaches copyright or IPR then it is illegal. Not every act of plagiarism is a breach of copyright. For example, you can plagiarise work that has no copyright.