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 into two distinct offenses: voluntary manslaughter and involuntary manslaughter, as defined under Illinois Compiled Statutes. Voluntary manslaughter, also known as second-degree murder in Illinois, occurs when an individual kills another without premeditation but does so in a sudden heat of passion prompted by provocation. Involuntary manslaughter, on the other hand, involves unintentionally killing another person due to acts that are likely to cause death or great bodily harm, or by recklessly performing acts that could cause such outcomes. Both offenses are considered felonies, with involuntary manslaughter typically carrying less severe penalties than voluntary manslaughter. The specific circumstances of the offense, such as the use of a motor vehicle or other aggravating factors, can influence the severity of the charges and the potential penalties upon conviction.