What is considered a murder of the first degree?

"Murder is the unlawful killing of a human being . . . with malice aforethought." ( 187, subd. (a).) "'Murder that is premeditated and deliberated is murder of the first degree.' [Citation.]" (People v. Pearson (2013) 56 Cal.4th 393, 443.) "'"'[P]remeditation' means thought over in advance,"' and '"'[d]eliberation' refers to careful weighing of considerations in forming a course of action . . . ."' [Citations.] 'An intentional killing is premeditated and deliberate if it occurred as the result of preexisting thought and reflection rather than unconsidered or rash impulse.' [Citation.]" (Ibid.) "'"Premeditation and deliberation can occur in a brief interval. 'The test is not time, but reflection. "Thoughts may follow each other with great rapidity and cold, calculated judgment may be arrived at quickly."'"' [Citation.]' [Citations.]" (People v. Solomon (2010) 49 Cal.4th 792, 812.) "[A] killing resulting from preexisting reflection, of any duration, is readily distinguishable from a killing based on unconsidered or rash impulse." (Id. at p. 813.)