The Collaborative International Dictionary of English v.0.48:

Fortuity \For*tu"i*ty\, n. [Cf. F. fortuit['e].]
   Accident; chance; casualty. --D. Forbes (1750).
   [1913 Webster]

WordNet (r) 3.0 (2006):

fortuity
    n 1: anything that happens suddenly or by chance without an
         apparent cause; "winning the lottery was a happy accident";
         "the pregnancy was a stroke of bad luck"; "it was due to an
         accident or fortuity" [syn: accident, stroke,
         fortuity, chance event]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "fortuity":
   accident, accidentality, actuarial calculation, adventitiousness,
   adventure, break, casualness, casualty, chance, chance hit,
   contingency, contingent, destiny, fate, fluke, flukiness,
   fortuitousness, fortune, freak accident, gamble, good fortune,
   good luck, hap, happening, happenstance, happy chance, hazard,
   heedless hap, how they fall, indeterminacy, indeterminateness,
   law of averages, long odds, long shot, lot, luck, lucky shot,
   moira, opportunity, principle of indeterminacy, probability,
   problematicness, random sample, risk, run of luck, serendipity,
   statistical probability, the breaks, theory of probability,
   uncertainty, uncertainty principle, whatever comes