A third-party defendant is a party who is brought into a lawsuit by an existing defendant, usually because the defendant believes that this new party is responsible for all or part of the plaintiff's claim. This mechanism allows the original defendant to seek indemnification or contribution from the third-party defendant, thus expanding the scope of the litigation to determine the respective liabilities among the parties involved. By introducing a third-party defendant, it can help resolve related claims in one action, promoting judicial efficiency.