what is a third party lawsuit

The general form of third party action is when a defendant sues someone else, not the plaintiff, in the same action.  For example, if there is a three car accident, and motorist A sues motorist B, claiming that motorist B was negligent.  Motorist B may assert a claim against motorist C alleging that the accident was motorist C's fault.  Another example, A loans money to B, guaranteed by C.  B defaults on payment, but A only sues C, the guarantor, because B is his friend.  C can assert a third party claim against B to be reimbursed for any money he has to pay to A.  Another - Patient A sues surgeon B because, A alleges, B's malpractice caused A to have a post-operative infection.  B thinks that C, the manufacturer of the sterilizing equipment used on the surgical equipment, because, B claims, it was defective and didn't sterilize the instruments correctly.  B therefore asserts a third party claim against C to be reimbursed for all, or part, of what he/she has to pay A.