Murder is the unlawful killing of another human without justification or valid excuse, especially the unlawful killing of another human being 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. 

Most societies consider murder to be a very serious crime, and thus believe that the person charged should receive harsh punishments for the purposes of retribution, deterrence, rehabilitation, or incapacitation. In most countries, a person convicted of murder generally faces a long-term prison sentence, possibly a life sentence; and in a few, the death penalty may be imposed. 

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 The elements of common law murder are:
Which jurist defined murder?
he eighteenth-century English jurist William Blackstone