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 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 an extremely serious crime, and thus that a person convicted of murder 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, a life sentence, or capital punishment.
The modern English word "murder" descends from the Proto-Indo-European *mŕ̥-trom which meant "killing", a noun derived from *mer- "to die".
Proto-Germanic in fact had two nouns derived from this word, later merging into the modern English noun: murþrą "death, killing, murder" (directly from Proto-Indo-Europeanmŕ̥-trom), whence Old English morðor "secret or unlawful killing of a person, murder; mortal sin, crime; punishment, torment, misery"; and *murþrijô "murderer; homicide" (from the verb *murþrijaną "to murder"), giving Old English myrþra "homicide, murder; murderer". There was a third word for "murder" in Proto-Germanic, continuing Proto-Indo-European *mr̥tós "dead" (compare Latin mors), giving Proto-Germanic *murþą "death, killing, murder" and Old English morþ "death, crime, murder" (compare German Mord).
The -d- first attested in Middle English mordre, mourdre, murder, murdre could have been influenced by Old French murdre, itself derived from the Germanic noun via Frankish *murþra (compare Old High German murdreo, murdiro), though the same sound development can be seen with burden (from burthen).