Source : Webster's Revised Unabridged Dictionary (1913)

Uncertainty \Un*cer"tain*ty\, n.; pl. {Uncertainties}.
   1. The quality or state of being uncertain.

   2. That which is uncertain; something unknown.

            Our shepherd's case is every man's case that quits a
            moral certainty for an uncertainty.   --L'Estrange.

Source : WordNet®

uncertainty
     n 1: being unsettled or in doubt; "the uncertainty of the
          outcome" [syn: {uncertainness}] [ant: {certainty}]
     2: the state of being unsure of something [syn: {doubt}, {incertitude},
         {dubiety}, {doubtfulness}, {dubiousness}] [ant: {certainty}]