Source : Webster's Revised Unabridged Dictionary (1913)

Improbability \Im*prob`a*bil"i*ty\, n.; pl. {Improbabilities}.
   [Cf. F. improbabilit['e].]
   The quality or state of being improbable; unlikelihood; also,
   that which is improbable; an improbable event or result.

Source : WordNet®

improbability
     n : the quality of being improbable [syn: {improbableness}]
         [ant: {probability}]