Heat of passion or sudden passion is any intense, emotional excitement that prompts violent and aggressive action, such as rage, anger, hatred, furious resentment, fright, or terror that would cause an ordinary person to act on impulse and without reflection. When a crime such as murder is committed in the heat of passion and in response to a provocation—as opposed to being premeditated or deliberated—the provocation may serve as a partial defense to the crime by eliminating the premeditation element of a murder charge (also known as malice or malice aforethought), reducing the charge to manslaughter, for example.

The classic example of heat of passion provocation is when a spouse or lover finds his or her partner in an intimate place or sexual act with another person and shoots, stabs, or otherwise kills or attempts to kill one or both of them. The heat of passion defense is only available when the defendant reacts immediately to the provocation and without cooling off. If, for example, the defendant leaves and returns 30 minutes later to kill the cheating spouse, the heat of passion defense may not be available.

And in some states the heat of passion defense may also apply when the defendant had the honest but unreasonable belief he was confronted with deadly force, and used deadly force to defend himself.

Laws regarding the availability of the heat of passion defense vary from state to state. Some state laws downgrade a murder charge to manslaughter when the crime was committed under the influence of extreme mental or emotional disturbance for which there is reasonable explanation or excuse. These laws are generally located in a state’s statutes—often in the penal or criminal code.

And even when a partial defense of heat of passion or extreme mental or emotional disturbance may be available to a defendant, a conviction for manslaughter can carry a lengthy prison term and other serious consequences.

In Washington State, the 'heat of passion' defense is recognized under certain circumstances. This defense can be invoked when a person commits a crime such as murder in response to a provocation that would cause an ordinary person to act impulsively and without reflection, driven by intense emotions like rage or terror. If successfully argued, this defense can reduce a murder charge to manslaughter by negating the element of premeditation or malice aforethought. However, for the defense to be applicable, the reaction must be immediate; there must not be a 'cooling off' period where the defendant had time to reflect on their actions. If there is a significant delay between the provocation and the act, the defense is less likely to succeed. Washington's laws on this matter are codified in the Revised Code of Washington (RCW), particularly in the sections dealing with homicide. While the heat of passion defense can mitigate the severity of charges, a manslaughter conviction still carries serious penalties, including a lengthy prison term.