noun (n)

• the quality of being improbable(noun.attribute)

  Example:
  Impossibility should never be confused with improbability.
  The improbability of such rare coincidences.
  

  Synonym:
  improbableness
  Antonym:
  probability
  

  source: wordnet30
• The quality or state of being improbable; unlikelihood; also, that which is improbable; an improbable event or result.(noun)

  source: webster1913