What is Onus Probandi?

Legal Definition
Evidence. The burden of the proof.

2. It is a general rule, that the party who alleges the affirmative of any proposition shall prove it. It is also a general rule that the onus probandi lies. upon the party who seeks to support his case by a particular fact of which he is supposed to be cognizant; for example, when to a plea of infancy, the plaintiff replies a promise after the defendant had attained his age, it is sufficient for the plaintiff to prove the promise and it lies on the defen-dant to show that he was not of age at the time. 1 Term. Rep. 648. But where the negative, involves a criminal omission by the party, and consequently where the law, by virtue of the general principle, presumes his innocence, the affirmative of the fact is also presumed. Vide 11 Johns. R. 513; 19 Johns. R. 345; 9 M. R. 48; 3 N. S. 576.

3. In general, wherever the law presumes the affirmative, it lies on the party who denies the fact, to prove the negative; as, when the law raises a presumption as to the continuance of life; the legitimacy of children born in wedlock; or the satisfaction of a debt. Vide. generally, 1 Phil. Ev. 156: 1 Stark. Ev. 376; Roscoe's Civ. Ev. 51 Roscoe's Cr. Ev. 55; B. P. 298; 2 Gall. 485; 1 McCord, 573; 12 Vin. Ab. 201; 4 Bouv. Inst. n. 4411.

4. The party on whom the onus probandi lies is entitled to begin, notwithstanding the technical form of the proceedings. 1 Stark. Ev. 584; 3 Bouv. last. n. 3043.
-- Bouviers Law Dictionary
Legal Definition
a Latin term for burden of proof.
Legal Definition
-- Ballentine's Law Dictionary