The Collaborative International Dictionary of English v.0.48:

Certainty \Cer"tain*ty\, n.; pl. Certainties. [OF.
   certainet['e].]
   1. The quality, state, or condition, of being certain.
      [1913 Webster]

            The certainty of punishment is the truest security
            against crimes. --Fisher Ames.
      [1913 Webster]

   2. A fact or truth unquestionable established.
      [1913 Webster]

            Certainties are uninteresting and sating. --Landor.
      [1913 Webster]

   3. (Law) Clearness; freedom from ambiguity; lucidity.
      [1913 Webster]

   Of a certainty, certainly.
      [1913 Webster]

WordNet (r) 3.0 (2006):

certainty
    n 1: the state of being certain; "his certainty reassured the
         others" [ant: doubt, doubtfulness, dubiety,
         dubiousness, incertitude, uncertainty]
    2: something that is certain; "his victory is a certainty" [syn:
       certainty, sure thing, foregone conclusion] [ant:
       precariousness, uncertainness, uncertainty]