Source : Webster's Revised Unabridged Dictionary (1913)

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

   {Onus probandi}[L.], obligation to furnish evidence to prove
      a thing; the burden of proof.

Source : WordNet®

onus
     n : an onerous or difficult concern; "the burden of
         responsibility"; "that's a load off my mind" [syn: {burden},
          {load}, {encumbrance}, {incumbrance}]