Manslaughter is the unlawful killing or homicide of a human being without the premeditation or malice (malice aforethought) required for the criminal offense of murder.

Many states have two types of manslaughter: (1) voluntary manslaughter—the defendant intended to kill the victim, but did so in self-defense, or in the heat of passion and without premeditation, or the defendant was insane and did not understand that the killing was wrong; and (2) involuntary manslaughter—the defendant negligently or recklessly caused the death of another person—such as when a person is intoxicated while operating a motor vehicle and causes an accident that results in the death of another person.

Some states do not have separate criminal offenses for voluntary and involuntary manslaughter—these states may have the offense of manslaughter and identify certain aggravating factors that may enhance the penalties upon a conviction for manslaughter.

Manslaughter is a felony offense and may be punished by a lengthy prison term. Manslaughter laws are generally located in a state's statutes—often in the penal or criminal code.

In California, manslaughter is categorized into voluntary and involuntary manslaughter, aligning with the general distinction made in many states. Voluntary manslaughter (California Penal Code Section 192(a)) occurs when a person kills another in the heat of passion or during a sudden quarrel without premeditation. It also covers killings that occur during a sudden combat or in the honest but unreasonable belief in the need to defend oneself. Involuntary manslaughter (California Penal Code Section 192(b)) involves the unintentional killing of another person due to reckless or negligent behavior that does not rise to the level of a felony but goes beyond ordinary negligence, such as a fatal accident caused by a misdemeanor act or lawful act done in an unlawful manner. Both types of manslaughter are considered felonies in California, with voluntary manslaughter carrying a potential prison sentence of 3 to 11 years, while involuntary manslaughter can result in 2 to 4 years in prison. California does not have a separate offense for manslaughter with aggravating factors; instead, such factors may be considered during sentencing or may lead to different charges such as murder under certain circumstances.