Wordnet 3.0
NOUN (1)
1. 
 anything that happens suddenly or by chance without an apparent cause; 
- Example: "winning the lottery was a happy accident"- Example: "the pregnancy was a stroke of bad luck"- Example: "it was due to an accident or fortuity"[syn: accident, stroke, fortuity, chance event]
WordNet (r) 3.0 (2006):
chance event
    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]