Onus – this term comes from from Latin for: the burden. This term is usually used in the context of evidence and refers both to the party with the burden (and to the scope of that burden), the latter depending whether the context is criminal or civil. Following the provisions of the Criminal Law in some legal systems, the onus of proof in criminal cases lies with the procecutor, who has the burden of proving beyond reasonable doubt. In civil cases, the onus of proof usually lies with the claimant, who has entered the claim and who must prove his statement by balance of probabilities.
See also http://thelawdictionary.org/onus