The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

improbability
    n 1: the quality of being improbable; "impossibility should
         never be confused with improbability"; "the improbability
         of such rare coincidences" [syn: improbability,
         improbableness] [ant: probability]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "improbability":
   abnormality, anomaly, bare possibility, conversation piece, curio,
   curiosity, dark horse, exception, fighting chance, gambling chance,
   hardly a chance, hundred-to-one shot, inexpectance, inexpectancy,
   inexpectation, little chance, little opportunity, long odds,
   long shot, museum piece, no expectation, nonesuch, nonexpectation,
   oddity, off chance, outside chance, poor bet, poor lookout,
   poor possibility, poor prospect, prodigiosity, prodigy, rarity,
   remote possibility, slim chance, small chance, strange thing,
   the unforeseen, the unlooked-for, unanticipation, unexpectedness,
   unforeseeableness, unlikelihood, unpredictability,
   unpredictableness, unpreparedness, unreadiness



The Devil's Dictionary (1881-1906):

IMPROBABILITY, n.

    His tale he told with a solemn face
    And a tender, melancholy grace.
        Improbable 'twas, no doubt,
        When you came to think it out,
        But the fascinated crowd
        Their deep surprise avowed
    And all with a single voice averred
    'Twas the most amazing thing they'd heard --
    All save one who spake never a word,
        But sat as mum
        As if deaf and dumb,
    Serene, indifferent and unstirred.
        Then all the others turned to him
        And scrutinized him limb from limb --
        Scanned him alive;
        But he seemed to thrive
        And tranquiler grow each minute,
        As if there were nothing in it.
    "What! what!" cried one, "are you not amazed
    At what our friend has told?"  He raised
    Soberly then his eyes and gazed
        In a natural way
        And proceeded to say,
    As he crossed his feet on the mantel-shelf:
    "O no -- not at all; I'm a liar myself."