A burden; an obligation.

Latin:" the burden". A term usually used in the context of evidence. The onus of proof in criminal cases lies with the state, in that it is the state that has the burden of proving beyond reasonable doubt. In civil cases, the onus of proof lies with the plaintiff who must prove his case by balance of probabilities. So "onus" can refer both to the party with the burden, and to the scope of that burden, the latter depending whether the context is criminal or civil.

an onerous or difficult concern; "the burden of responsibility"; "that's a load off my mind"

O EPB/ US