Is copying the exact statement of a definition or a theorem considered plagiarism?
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?