Is copying the exact statement of a definition or a theorem considered plagiarism? Let's say in a paper when I am talking about a theorem someone else proved, I state the result in the exact same way, word for word, as in their paper. Is this considered plagiarism? I would still give credit and citation to the result, just that I don't paraphrase the statement.
In mathematics, often there are highly optimized/perfected statements of theorems. It would be silly to alter them (introducing damage?!) just for the sake of avoiding exact quoting. Cite, that's all.
That is, unless you have something to add to their idea, there's scant point to changing the wording... apart from the risk of mis-stating them!... just to meet a sort of fake goal. Cite and acknowledge. Be honest. With citation, what could possibly be the objection to quoting a perfected assertion of a good theorem?