What is the proper named for this kind of defendant in a counter-claim?

I just wanted to know the proper term. If the defendant files a counter-claim the plaintiffs become counter-defendant but if there is one person that was never part of the original suit and the defendant also wants to sue them along with plaintiffs what is that person called. The plaintiffs are counter-defendants but if there is one new party that was not in the original suit, what are they called? Sorry I know my question is the best. Are they a third-party, counter-defendant,