The reply is a response by plaintiff

Plaintiff

A plaintiff , also known as a claimant or complainant, is the term used in some jurisdictions for the party who initiates a lawsuit before a court...

 to defendant

Defendant

A defendant or defender is any party who is required to answer the complaint of a plaintiff or pursuer in a civil lawsuit before a court, or any party who has been formally charged or accused of violating a criminal statute...

's answer

Answer

Generally, an answer is a reply to a question or is a solution, a retaliation, or a response that is relevant to the said question.In law, an answer was originally a solemn assertion in opposition to some one or something, and thus generally any counter-statement or defense, a reply to a question...

. A reply occurs only when defendant has asserted a counterclaim or the court has ordered a reply.

It is important to keep in mind that "plaintiff" in this context may also refer to an impleaded party. So, if a defendant impleads a party, this new party is the third-party defendant and the original defendant is the third-party plaintiff. The third-party plaintiff must file a complaint

Complaint

In legal terminology, a complaint is a formal legal document that sets out the facts and legal reasons that the filing party or parties In legal terminology, a complaint is a formal legal document that sets out the facts and legal reasons (see: cause of action) that the filing party or parties In...

on the third-party defendant, who then must answer. The court may order a reply to this third-party defendant's answer.