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) 9A.32. Manslaughter in the first degree (RCW 9A.32.060) is when a person recklessly causes the death of another person, which may be akin to involuntary manslaughter. Manslaughter in the second degree (RCW 9A.32.070) occurs when a person, with criminal negligence, causes the death of another person. This can be compared to voluntary manslaughter in other jurisdictions, although Washington law does not explicitly use the terms 'voluntary' or 'involuntary.' Both degrees of manslaughter are considered felonies in Washington, with first-degree manslaughter being a more serious offense than second-degree manslaughter. The penalties upon conviction can include substantial prison time, with the exact sentence depending on factors such as the defendant's criminal history and the circumstances of the offense.