A demurrer is a pleading in a lawsuit that objects to or challenges a pleading filed by an opposing party. The word demur means "to object"; a demurrer is the document that makes the objection. Lawyers informally define a demurrer as a defendant saying "So what?" to the pleading.
Typically, the defendant in a case will demur to the complaint, but it is also possible for the plaintiff to demur to an answer. The demurrer challenges the legal sufficiency of a cause of action in a complaint or of an affirmative defense in an answer. If a cause of action in a complaint does not state a cognizable claim or if it does not state all the required elements, then the challenged cause of action or possibly the entire complaint can be thrown out (informally speaking) at the demurrer stage as not legally sufficient. A demurrer is typically filed near the beginning of a case in response to the plaintiff filing a complaint or the defendant answering the complaint.
In common law, a demurrer was the pleading through which a defendant challenged the legal sufficiency of a complaint in criminal or civil cases. Today, however, the pleading has been discontinued in many jurisdictions, including the United Kingdom, the U.S. federal court system, and most U.S. states (though some states, including California, Pennsylvania, and Virginia, retain it). In criminal cases, a demurrer was considered a common law due process right, to be heard and decided before the defendant was required to plead "not guilty," or make any other pleading in response, without having to admit or deny any of the facts alleged.
A demurrer generally assumes the truth of all material facts alleged in the complaint, and the defendant cannot present evidence to the contrary, even if those facts appear to be obvious fabrications by the plaintiff or are likely to be easily disproved during litigation. That is, the point of the demurrer is to test whether a cause of action or affirmative defense as pleaded is legally insufficient, even if all facts pleaded are assumed to be true.
The sole exception to the no-evidence rule is that a court may take judicial notice of certain things. For example, the court can take judicial notice of commonly known facts not reasonably subject to challenge, such as the Gregorian calendar, or of public records, such as a published legislative report showing the intent of the legislature in enacting a particular statute.
Overview
Civil cases
A demurrer is commonly filed by a defendant in response to a complaint filed by the plaintiff. A demurrer to a complaint can terminate a lawsuit. Although a plaintiff may demur to a defendant's answer to a complaint or the defendant's affirmative defenses, a demurrer to an answer is less common because it may be a poor strategic move. A demurrer to an answer may simplify a lawsuit, but it usually will not end the lawsuit; it is normally used only when the plaintiff intends to move for summary judgment in their favor at the earliest opportunity and needs to preemptively attack some of the defendant's affirmative defenses.
Technically, a demurrer is not a motion; a party does not file a motion for demurrer nor move the court to demur. Rather, a demurrer is a particular type of pleading and demurring is the act by which a party formally requests the court to dismiss a cause of action (claim) or the entire complaint.
In lay terms, a judge who sustains a demurrer is saying that the law does not recognize a legal claim for the facts stated by the complaining party. If the judge overrules a demurrer, the court is allowing the claim or case to proceed.
In legal terms, the demurring party asserts that the complaint or counterclaim does not amount to a legally valid claim, even if the factual allegations contained in the complaint or counterclaim are accepted as true.
Usually, a demurrer attacks a complaint as missing one or more required elements of a claim. Those elements are usually attacked by showing that the plaintiff failed to plead an essential element per se or facts that adequately support it (e.g., facts giving rise to an actionable duty running from the defendant to the plaintiff). Another method is to attack the entire cause of action itself as abolished or prohibited as against public policy (e.g., wrongful life is against public policy in most jurisdictions).