Not sure what you're asking, here. You mean the term? What does it mean?

To my mind, it's a homicide, the killing of a person, with justification. For example, if your life is threatened, you have the right to defend yourself, even if that results in the death of your attacker. Killing in self-defense is justifiable homicide. (Based on several discussions I had in the past with MMI, I'm not certain that this is true in the UK, though.)