What if I use an item from another story in my story? For example, in my story there is a metal called adamantium which obviously came from the story of Wolverine.
Can it be considered plagiarism?
The short answer is yes. And what's more you will be facing hefty and quick legal action from Marvel/Disney for utilizing their IP.
Although if just using this one element, it's not technically plagiarism, it is copyright infringement, and that's potentially a lot more expensive.
That is because you are taking an original creation and stealing it for your work. It would be the same if you were to incorporate Anduril or Superman or Kryptonite, or X-Wings.
I would say your pretty safe using the word Adamantium. Specially since you have Adamantine (real) and Adamantite (fictional) as metals all derived from the word Adamant. All 3 words could be switched around without too much effect.
The word Adamantium also appears in Warhammer and Maplestory which are pretty large games.