Plagiarism is passing someone else's work off as your own. Unauthorized copying is just providing a work for others to consume without claiming credit for it, but without having a legal right to do so. Plagiarism doesn't really relate to authorization at all. If you have permission from someone to put your name on their work, that doesn't change the fact that you are lying, which can have consequences of its own (especially in an academic context).

I thought this was obvious, but maybe it isn't? I see disclaimers on Youtube all the time ("I don't own this") which seem to imply the person posting the video doesn't understand the difference.