A rule of evidence that imposes on a participant in a court case the initial obligation to prove a certain thing or the contrary will be assumed by the court.
For example, in criminal trials, the prosecution has the burden of proving the accused guilt because innocence is presumed.
In Sellers v. Kincaid 303 Ill 216 (Illinois Supreme Court, 1922):
"Burden of proof has two distinct meanings; that generally the burden of proof, in the sense of producing evidence, passes from party to party as the case progresses, while the burden of proof in the sense of the obligation to establish the truth of the claim by a preponderance of the evidence rests throughout upon the party asserting the affirmative of the issue, and that, in the contest of a will, the burden of proof is on the proponent to prove the will."