The modern English word "murder" descends from the Proto-Indo-European "mrtró" which meant "to die". The Middle English mordre is a noun from Anglo-Saxon morðor and Old French murdre. Middle English mordre is a verb from Anglo-Saxon myrdrian and the Middle English noun.

The eighteenth-century English jurist William Blackstone (citing Edward Coke), in his Commentaries on the Laws of England set out the common law definition of murder, which by this definition occurs

when a person, of sound memory and discretion, unlawfully kills any reasonable creature in being and under the king's peace, with malice aforethought, either express or implied.

The elements of common law murder are:

The Unlawful – This distinguishes murder from killings that are done within the boundaries of law, such as capital punishment, justified self-defence, or the killing of enemy combatants by lawful combatants as well as causing collateral damage to non-combatants during a war.

Killing – At common law life ended with cardiopulmonary arrest – the total and irreversible cessation of blood circulation and respiration. With advances in medical technology courts have adopted irreversible cessation of all brain function as marking the end of life.

?riminal act or omission – Killing can be committed by an act or an omission.

of a human – This element presents the issue of when life begins. At common law, a fetus was not a human being. Life began when the fetus passed through the vagina and took its first breath.

by another human – In early common law, suicide was considered murder. The requirement that the person killed be someone other than the perpetrator excluded suicide from the definition of murder.

with malice aforethought – Originally malice aforethought carried its everyday meaning – a deliberate and premeditated (prior intent) killing of another motivated by ill will. Murder necessarily required that an appreciable time pass between the formation and execution of the intent to kill. The courts broadened the scope of murder by eliminating the requirement of actual premeditation and deliberation as well as true malice. All that was required for malice aforethought to exist is that the perpetrator act with one of the four states of mind that constitutes "malice".

The four states of mind recognized as constituting "malice" are:

Under state of mind (i), intent to kill, the deadly weapon rule applies. Thus, if the defendant intentionally uses a deadly weapon or instrument against the victim, such use authorizes a permissive inference of intent to kill. In other words, "intent follows the bullet". Examples of deadly weapons and instruments include but are not limited to guns, knives, deadly toxins or chemicals or gases and even vehicles when intentionally used to harm one or more victims.

Under state of mind (iii), an "abandoned and malignant heart", the killing must result from the defendant's conduct involving a reckless indifference to human life and a conscious disregard of an unreasonable risk of death or serious bodily injury. In Australian jurisdictions, the unreasonable risk must amount to a foreseen probability of death (or grievous bodily harm in most states), as opposed to possibility.

Under state of mind (iv), the felony-murder doctrine, the felony committed must be an inherently dangerous felony, such as burglary, arson, rape, robbery or kidnapping. Importantly, the underlying felony cannot be a lesser included offense such as assault, otherwise all criminal homicides would be murder as all are felonies.

As with most legal terms, the precise definition of murder varies between jurisdictions and is usually codified in some form of legislation. Even when the legal distinction between murder and manslaughter is clear, it is not unknown for a jury to find a murder defendant guilty of the lesser offence. The jury might sympathise with the defendant (e.g. in a crime of passion, or in the case of a bullied victim who kills their tormentor), and the jury may wish to protect the defendant from a sentence of life imprisonment or execution.

Degrees of murder

Many jurisdictions divide murder by degrees. The distinction between first- and second-degree murder exists, for example, in Canadian murder law and U.S. murder law.

The most common division is between first- and second-degree murder. Generally, second-degree murder is common law murder, and first-degree is an aggravated form. The aggravating factors of first-degree murder depend on the jurisdiction, but may include a specific intent to kill, premeditation, or deliberation. In some, murder committed by acts such as strangulation, poisoning, or lying in wait are also treated as first-degree murder. A few states in the U.S. further distinguish third-degree murder, but they differ significantly in which kinds of murders they classify as second-degree versus third-degree. For example, Minnesota defines third-degree murder as depraved-heart murder, whereas Florida defines third-degree murder as felony murder (except when the underlying felony is specifically listed in the definition of first-degree murder).

Some jurisdictions also distinguish premeditated murder. This is the crime of wrongfully and intentionally causing the death of another human being (also known as murder) after rationally considering the timing or method of doing so, in order to either increase the likelihood of success, or to evade detection or apprehension.State laws in the United States vary as to definitions of "premeditation". In some states, premeditation may be construed as taking place mere seconds before the murder. Premeditated murder is one of the most serious forms of homicide, and is punished more severely than manslaughter or other types of murder, often with a life sentence without the possibility of parole, or in some countries, the death penalty. In the U.S, federal law (18 U.S.C. § 1111(a)) criminalizes premeditated murder, felony murder and second-degree murder. In Canada, the Criminal Code classifies murder as either 1st- or 2nd-degree. The former type of murder is often called premeditated murder, although premeditation is not the only way murder can be classified as first-degree.

Common law

According to Blackstone, English common law identified murder as a public wrong. According to common law, murder is considered to be malum in se, that is an act which is evil within itself. An act such as murder is wrong or evil by its very nature. And it is the very nature of the act which does not require any specific detailing or definition in the law to consider murder a crime.

Some jurisdictions still take a common law view of murder. In such jurisdictions, what is considered to be murder is defined by precedent case law or previous decisions of the courts of law. However, although the common law is by nature flexible and adaptable, in the interests both of certainty and of securing convictions, most common law jurisdictions have codified their criminal law and now have statutory definitions of murder.

Exclusions

General

Although laws vary by country, there are circumstances of exclusion that are common in many legal systems.

• Killing of enemy combatants who have not surrendered by lawful combatants, in accordance with lawful orders in war, is also generally not considered murder; although illicit killings within a war may constitute murder or homicidal war crimes. (see the Laws of war article)
• Self-defence: acting in self-defence or in defence of another person is generally accepted as legal justification for killing a person in situations that would otherwise have been murder. However, a self-defence killing might be considered manslaughter if the killer established control of the situation before the killing took place. In the case of self-defence it is called a "justifiable homicide".
• Unlawful killings without malice or intent are considered manslaughter.
• In many common law countries, provocation is a partial defence to a charge of murder which acts by converting what would otherwise have been murder into manslaughter (this is voluntary manslaughter, which is more severe than involuntary manslaughter).
• Accidental killings are considered homicides. Depending on the circumstances, these may or may not be considered criminal offenses; they are often considered manslaughter.
• Suicide does not constitute murder in most societies. Assisting a suicide, however, may be considered murder in some circumstances.

Specific to certain countries

• Capital punishment: some countries practice the death penalty. Capital punishment may be ordered by a legitimate court of law as the result of a conviction in a criminal trial with due process for a serious crime. The 47 Member States of the Council of Europe are prohibited from using the death penalty.
• Euthanasia, doctor-assisted suicide: the administration of lethal drugs by a doctor to a terminally ill patient, if the intention is solely to alleviate pain, in many jurisdictions it is seen as a special case (see the doctrine of double effect and the case of Dr John Bodkin Adams).
• Killing to prevent the theft of one's property may be legal, depending on the jurisdiction. In 2013, a jury in south Texas acquitted a man who killed a prostitute who attempted to run away with his money.
• Killing an intruder who is found by an owner to be in the owner's home (having entered unlawfully): legal in most US states (see Castle doctrine).
• Killing to prevent specific forms of aggravated rape or sexual assault – killing of attacker by the potential victim or by witnesses to the scene; legal in parts of the US and in various other countries.
• In Pakistan, the killing of a woman or girl in specific circumstances (e.g., when she commits adultery and is killed by her husband or other family members, known as honor killing) is not considered murder.
• In the United States, in some states and in federal jurisdiction, a killing by a police officer is excluded from prosecution if the officer believes they are being threatened with deadly force by the victim. This may include such actions by the victim as reaching into a glove compartment or pocket for license and registration, if the officer thinks that the victim might be reaching for a gun.
• Space jurisdiction is similar to that of international waters. Therefore, a murder committed in outer space is subject to jurisdiction in the country that owns the space craft in which the killing transpired. In the event the murder occurred on an extraterrestrial planet (e.g. The Moon), no country can own land of any other planet so the killer is bound by the laws of the country in which they originate. This also applies to the ISS per agreement signed by all countries that have worked on the station so all astronauts are covered by extraterratorial jurisdiction.

Victim

All jurisdictions require that the victim be a natural person; that is, a human being who was still alive before being murdered. In other words, under the law one cannot murder a corpse, a corporation, a non-human animal, or any other non-human organism such as a plant or bacterium.

California's murder statute, Penal Code Section 187, was interpreted by the Supreme Court of California in 1994 as not requiring any proof of the viability of the fetus as a prerequisite to a murder conviction. This holding has two implications. The first is a defendant in California can be convicted of murder for killing a fetus which the mother herself could have terminated without committing a crime. The second, as stated by Justice Stanley Mosk in his dissent, is that because women carrying nonviable fetuses may not be visibly pregnant, it may be possible for a defendant to be convicted of intentionally murdering a person he did not know existed.

Mitigating circumstances

Some countries allow conditions that "affect the balance of the mind" to be regarded as mitigating circumstances. This means that a person may be found guilty of "manslaughter" on the basis of "diminished responsibility" rather than being found guilty of murder, if it can be proved that the kil