Murder occurs when one human being unlawfully kills another human being. See Homicide. The precise legal definition of murder varies by jurisdiction. Most states distinguish between different degrees of murder. Some other states base their murder laws on the Model Penal Code.

At common law, murder was defined as killing another human being with malice aforethought. Malice aforethought is a legal term of art, that encompasses the following types of murder:

• "Intent-to-kill murder"

• "Grievous-bodily-harm murder" - Killing someone in an attack intended to cause them grievous bodiliy harm. For example, if a person fatally stabbed someone, even if she only intended to wound her victim, she could still be executed.

• "Felony-murder" - Killing someone while in the process of committing a felony. Note that at common law, there were few felonies, and all carried the death penalty. For example, at common law, robbery was a felony. So if a robber accidentally killed someone during a robbery, the robber could be executed.

• "Depraved heart murder" - Killing someone in a way that demonstrates a callous disregard for the value of human life. For example, if a person intentionally fires a gun into a crowded room, and someone dies, the person could be convicted of depraved heart murder.