A defendant is generally the party against whom a lawsuit is initiated or a claim is asserted. They are the person or entity from whom the plaintiff demands a legal right. The complaint filed by the plaintiff alleges a cause of action arising from the defendant's act or omission that violated the plaintiff's legal right.

In an action in personam, the defendant is the person against whom the action is directed, seeking to hold them personally liable for a sum of money, delivery of a thing, or performance/non-performance of an act. The judgment in such an action binds only the parties properly impleaded. Jurisdiction over the person of the defendant is typically required in in personam actions to impose personal responsibility or liability. This jurisdiction is acquired either through valid service of summons or the defendant's voluntary appearance in court. Voluntary appearance is equivalent to service of summons, but including other grounds besides lack of jurisdiction over the person in a motion to dismiss or answer can be deemed voluntary submission. If the defendant's identity is unknown, they may be sued under a designation like "unknown owner," with the pleading amended once their name is discovered. An entity without juridical personality can be sued under its common name, but its responsive pleading must disclose the names and addresses of its members.

In actions in rem or quasi in rem, such as those affecting property title or an individual's interest in property, jurisdiction over the person of the defendant is not always a prerequisite for the court to have jurisdiction over the subject matter (res or property), although service of summons is still required to satisfy due process. In a quasi in rem action, an individual is specifically named as the defendant, and the goal is to subject their interest in the property to an obligation or lien burdening it.

The primary responsive pleading for a defendant is an answer, where they set forth their defenses. Failure to file an answer within the required period can result in a declaration of default. A defendant can also assert claims against the plaintiff through a counterclaim, against a co-defendant through a cross-claim, or against a third party through a third-party complaint, thereby becoming a claiming party. In expropriation cases, the defendant must serve an answer identifying their property interest and stating objections to the taking, though they can still present evidence on just compensation even without a prior answer. Affirmative defenses not pleaded in the answer are generally waived. A defendant may file a motion to dismiss if the plaintiff lacks legal capacity to sue or if the complaint fails to state a cause of action. A defendant may also file a demurrer to evidence after the plaintiff rests, arguing the plaintiff's evidence is legally insufficient. In criminal proceedings, the defendant is the accused person.