What is "imperfect self-defense"?

"Imperfect self-defense is the killing of another human being under the actual but unreasonable belief that the killer was in imminent danger of death or great bodily injury." (People v. Booker (2011) 51 Cal.4th 141, 182.) Imperfect self-defense is similar to, but distinguishable from, true or perfect self-defense. (People v. Barton (1995) 12 Cal.4th 186, 199.) "The sole difference between true self-defense and 'unreasonable self-defense' is that the former applies only when the defendant acts in response to circumstances that cause the defendant to fear, and would lead a reasonable person to fear, the imminent infliction of death or great bodily injury ( 197, 198); unreasonable self-defense, on the other hand, does not require the defendant's fear to be reasonable. [Citation.]" (Id. at pp. 199-200.)