California Code, Evidence Code - EVID § 115

“Burden of proof” means the obligation of a party to establish by evidence a requisite degree of belief concerning a fact in the mind of the trier of fact or the court. The burden of proof may require a party to raise a reasonable doubt concerning the existence or nonexistence of a fact or that he establish the existence or nonexistence of a fact by a preponderance of the evidence, by clear and convincing proof, or by proof beyond a reasonable doubt.

Except as otherwise provided by law, the burden of proof requires proof by a preponderance of the evidence.