The Collaborative International Dictionary of English v.0.48:

Onus \O"nus\, n. [L.]
   A burden; an obligation.
   [1913 Webster]

   Onus probandi[L.], the obligation to furnish evidence to
      prove an assertion; the burden of proof.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ONUS PROBANDI, 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 defendant 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.