Most definitions of plagiarism describe it as the theft of someone's words or ideas.
At the same time, most of them aren't as clear that plagiarism is a kind of fraud. Fraud is misrepresentation that aims to gain an unfair advantage.
Since many cases of plagiarism don't involve any actual theft, and all of them involve misrepresentation, fraud seems to be a better description of what's wrong with plagiarism.
Plagiarism is the act of representing intellectual work as something it is not -- including representing someone else's work as your own and representing your own previous work as something new.