Source : Webster's Revised Unabridged Dictionary (1913)

Certainty \Cer"tain*ty\, n.; pl. {Certainties}. [OF.
   certainet['e].]
   1. The quality, state, or condition, of being certain.

            The certainty of punishment is the truest security
            against crimes. --Fisher Ames.

   2. A fact or truth unquestionable established.

            Certainties are uninteresting and sating. --Landor.

   3. (Law) Clearness; freedom from ambiguity; lucidity.

   {Of a certainty}, certainly.

Source : WordNet®

certainty
     n 1: the state of being certain; "his certainty reassured the
          others" [ant: {doubt}]
     2: something that is certain; "his victory is a certainty"
        [syn: {sure thing}, {foregone conclusion}] [ant: {uncertainty}]