Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

fortuity
     n : anything that happens by chance without an apparent cause
         [syn: {accident}, {chance event}]