Murder is the unlawful killing of another human without justification or valid excuse, especially the unlawful killing of another human with malice aforethought. This state of mind may, depending upon the jurisdiction, distinguish murder from other forms of unlawful homicide, such as manslaughter. Manslaughter is a killing committed in the absence of malice, brought about by reasonable provocation, or diminished capacity. Involuntary manslaughter, where it is recognized, is a killing that lacks all but the most attenuated guilty intent, recklessness.

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.

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 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.

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 they did not know existed.

For a killing to be considered murder in nine out of fifty states in the US, there normally needs to be an element of intent. A defendant may argue that they took precautions not to kill, that the death could not have been anticipated, or was unavoidable. As a general rule, manslaughter constitutes reckless killing, but manslaughter also includes criminally negligent (i.e. grossly negligent) homicide. Unintentional killing that results from an involuntary action generally cannot constitute murder. After examining the evidence, a judge or jury (depending on the jurisdiction) would determine whether the killing was intentional or unintentional.

A legal doctrine in some common law jurisdictions broadens the crime of murder: when an offender kills in the commission of a dangerous crime, (regardless of intent), he/she is guilty of murder. The felony murder rule is often justified by its supporters as a means of deterring dangerous felonies, but the case of Ryan Holle shows it can be used very widely.

The World Health Organization reported in October 2002 that a person is murdered every 60 seconds. An estimated 520,000 people were murdered in 2000 around the globe. Another study estimated the worldwide murder rate at 456,300 in 2010 with a 35% increase since 1990. Two-fifths of them were young people between the ages of 10 and 29 who were killed by other young people. Because murder is the least likely crime to go unreported, statistics of murder are seen as a bellwether of overall crime rates.

In the United States, 666,160 people were killed between 1960 and 1996. Approximately 90% of murders in the US are committed by males. Between 1976 and 2005, 23.5% of all murder victims and 64.8% of victims murdered by intimate partners were female. For women in the US, homicide is the leading cause of death in the workplace.

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