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 Maine, manslaughter is defined under Title 17-A, Chapter 9 of the Maine Criminal Code. The state recognizes both voluntary and involuntary manslaughter. Voluntary manslaughter in Maine occurs when a person intentionally or knowingly causes the death of another under circumstances that do not constitute murder because of extreme anger or fear brought about by adequate provocation. Involuntary manslaughter, referred to as 'manslaughter' in Maine statutes, occurs when a person recklessly, or with criminal negligence, causes the death of another person. This can include situations such as vehicular manslaughter where a person's operation of a motor vehicle under the influence results in a fatality. Manslaughter in Maine is a Class A crime, which is punishable by up to 30 years of imprisonment and significant fines. The state does not categorize manslaughter with separate offenses but does consider aggravating factors that may enhance penalties upon conviction.