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 Washington State, manslaughter is categorized into two degrees as defined under the Revised Code of Washington (RCW). First-degree manslaughter (RCW 9A.32.060) occurs when a person recklessly causes the death of another, reflecting elements of involuntary manslaughter. Second-degree manslaughter (RCW 9A.32.070) involves causing the death of another person due to criminal negligence, which also aligns with the concept of involuntary manslaughter. Washington does not have a separate statute for voluntary manslaughter; however, a killing that might be considered voluntary manslaughter in other states could be prosecuted under different charges such as second-degree murder (RCW 9A.32.050) if there was intent but without premeditation, or potentially as a lesser included offense during a murder trial. Manslaughter in Washington is a serious felony offense and can result in substantial prison time, with the specific penalties determined by the state's sentencing guidelines and the circumstances of the case.