Typically, a breach of contract case involves two parties:
- The plaintiff is the party who files suit and
- The defendant, the party against whom the suit is filed.
In this hypothetical, Peter is the plaintiff and Daniel is the defendant.
In some breach of contract cases, a defendant's best defense is that someone who isn't named in the original lawsuit is liable. This category of "people who could be responsible but who aren't either a plaintiff or defendant in the original lawsuit" is known generally as "third parties" (because they are neither the "first party," or plaintiff, nor the "second party," or defendant).
Florida Rule of Civil Procedure 1.180(a) states, in part:
At any time after commencement of the action a defendant may have a summons and complaint served on a person not a party to the action who is or may be liable to the defendant for all or part of the plaintiff’s claim against the defendant, and may also assert any other claim that arises out of the transaction or occurrence that is the subject matter of the plaintiff’s claim.
Here, Daniel has filed an action against Connor on the grounds that Connor will be liable to Daniel for part or all of what Daniel will owe Peter if Peter wins the suit he filed against Daniel. Connor is a "third party" in relationship to Peter and Daniel's original dispute. So, Daniel's suing of Connor is a third party claim.
A common mistake is confusing third-party claims with cross-claims. The primary difference is that in a cross-claim, one defendant (or plaintiff) sues another person who is already named in the lawsuit.
This hypothetical would be a cross-claim, for instance, if Peter had named both Daniel and Connor as defendants, and then Daniel had sued Connor.
Because Peter did not name Connor as a defendant—Connor only came in later when Daniel sued him—the Daniel-Connor claim is a third-party claim.