noun (n)

• an unfortunate mishap; especially one causing damage or injury(noun.event)

  source: wordnet30
• anything that happens suddenly or by chance without an apparent cause(noun.event)

  Example:
  Winning the lottery was a happy accident.
  It was due to an accident or fortuity.
  

  Synonym:
  chance event, fortuity, stroke
  

  source: wordnet30
• Literally, a befalling; an event that takes place without one's foresight or expectation; an undesigned, sudden, and unexpected event; chance; contingency; often, an undesigned and unforeseen occurrence of an afflictive or unfortunate character; a casualty; a mishap; as, to die by an accident.(noun)

  source: webster1913