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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 is also used to reference 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, the pleading that initiates a lawsuit. 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; statutory causes of action; torts such as assault, battery, invasion of privacy, fraud, slander, negligence, intentional infliction of emotional distress; and suits in equity 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 insufficient information to form a response. The answer may also contain counterclaims in which the "Counterclaim Plaintiff" states its own causes of action. Finally, the answer may contain affirmative defenses. Most defenses must be raised at the first possible opportunity either in the answer or by motion or are deemed waived. A few defenses, in particular a court's lack of subject matter jurisdiction, need not be plead and may be raised at any time.