What is "perfect self-defense" and what is the test for this?

^4. "[I]mperfect self-defense occurs when a defendant acts in the actual but unreasonable belief that he or she is in imminent danger of great bodily injury or death." (People v. Simon (2016) 1 Cal.5th 98, 132.) "Because imperfect self-defense reduces an intentional, unlawful killing from murder to voluntary manslaughter by negating the element of malice, this form of voluntary manslaughter is considered a lesser and necessarily included offense of murder." (Ibid.)