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 Illinois, manslaughter is categorized under the broader term 'homicide' and is divided into two main types: voluntary manslaughter and involuntary manslaughter. Voluntary manslaughter in Illinois is referred to as 'second-degree murder' under 720 ILCS 5/9-2. It occurs when an individual kills another without premeditation but does so in a sudden heat of passion prompted by adequate provocation, or under an unreasonable but honest belief that circumstances existed that would justify deadly force. Involuntary manslaughter, as defined by 720 ILCS 5/9-3, occurs when an individual unintentionally kills another person without lawful justification by acts that are likely to cause death or great bodily harm, or by recklessly performing acts that are likely to cause death or great bodily harm to another. Both offenses are felonies, with second-degree murder carrying a more severe penalty than involuntary manslaughter. The specific penalties for these offenses are determined by a variety of factors, including the presence of aggravating circumstances, which can enhance the penalties upon conviction.