In the law, a cause of action (sometimes called a claim) is a set of facts sufficient to justify a right to sue. The phrase may refer to the legal theory upon which a plaintiff brings suit (such as breach of contract, battery, or false imprisonment).
To pursue a cause of action, a plaintiff pleads or alleges facts in a complaint. A cause of action generally encompasses both the legal theory (the legal wrong the plaintiff claims to have suffered) and the remedy (the relief a court is asked to grant). Often the facts or circumstances that entitle a person to seek judicial relief may create multiple causes of action.
There are a number of specific causes of action, including: contract-based actions (such as breach of contract and breach of the implied covenant of good faith and fair dealing); statutory causes of action (based upon the violation of a statute); torts such as assault, battery, invasion of privacy, fraud, slander, negligence, intentional infliction of emotional distress; and suits in equity (based on fairness) such as unjust enrichment and quantum meruit.
The points a plaintiff must prove to win a given type of case are called the "elements" of that cause of action. For example, for a claim of negligence, the elements are: the (existence of a) duty, breach (of that duty), proximate cause (by that breach), and damages. If a complaint does not allege facts sufficient to support every element of a claim, the court, upon motion by the opposing party, may dismiss the complaint for failure to state a claim for which relief can be granted.
The defendant to a cause of action must file an "Answer" to the complaint in which the claims can be admitted, denied, or stated that insufficient information exists to form a response. In Nevada, the answer must be filed within 20 days of the date of the service of the complaint upon the defendant. The answer may also contain counterclaims in which the defendant (called "Counter Claimant") states its own causes of action against the plaintiff (called "Counter Defendant"). Finally, the answer may contain affirmative defenses.
An affirmative defense is a category of defense used in litigation between private parties in common law jurisdictions. Affirmative defense can be classified as either a justification defense or an excuse defense. Affirmative defenses operate to limit, excuse or avoid a defendant's civil liability, even though the factual allegations of plaintiff's claim are admitted or proven.
In a criminal case, a clear illustration of an affirmative defense is self-defense. In its simplest form, a criminal defendant may be exonerated, if he or his lawyer can demonstrate that he had an honest and reasonable belief that his conduct was necessary to protect himself against another's use of unlawful force.
Most affirmative defenses must be timely pleaded by a defendant in order for the court to consider them, or else they are considered waived by the defendant's failure to assert them. The classic unwaivable affirmative defense is lack of subject-matter jurisdiction. What constitutes timely assertion is often itself the subject of contentious litigation.
Because an affirmative defense requires an assertion of facts beyond those claimed by the plaintiff, generally the party making an affirmative defense bears the burden of proof. The burden of proof is typically lower than beyond a reasonable doubt. It can either be proof by clear and convincing evidence or a preponderance of the evidence. In some cases or jurisdictions, however, the defense must only be asserted, and the prosecution has the burden of proving beyond a reasonable doubt that the defense is not applicable.
Rule 8 of the Federal Rules of Civil Procedure governs the assertion of affirmative defenses in civil cases filed in the United States district courts. Rule 8(c) specifically enumerates the following defenses: "accord and satisfaction, arbitration and award, assumption of risk, contributory negligence, discharge in bankruptcy, duress, estoppel, failure of consideration, fraud, illegality, injury by fellow servant, laches, license, payment, release, res judicata, statute of frauds, statute of limitations, waiver, and any other matter constituting an avoidance or affirmative defense."