Copyright Vs. Plagiarism
Copyright is different from plagiarism - plagiarism is an appropriation of someone else's work without giving proper credit. You can plagiarize without infringing a copyright.
Example: Shakespeare's works are out of copyright because he's been dead more than 70 years. So you cannot violate his copyright. However, if you were to try to pass off one of his sonnets as your own, you would be plagiarizing.