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?
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.
Yes it could be considered plagiarism, but not under copyright infringement like your tags suggest. This would be an infringement on trademark. Copyright infringement would come from if you used the same story behind the metal, then that would be copyright infringement.
The trick here, if you want to avoid any risk of it being claimed as an infringement, is to just not use the same name if the base concept is common enough. For example, adamantium is just a REALLY durable metal. This concept has been around for hundreds if not thousands of years. Now, a pen that becomes a sword when uncapped however, that would be plagiarism even with a name change because there are enough layers to that which makes it specific and unique. A sword of energy that can cut through anything in front of it? Could be a lightsaber... or it could be the "flaming sword" wielded by the angel outside the gates of the Garden of Eden. I think you can piece together from here how to figure out what is and isn't potentially safe... but either way:
I am not a lawyer, so don't think of what I am saying to be legal advice please. Thank you!
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.