noun (n)

• incapability of existing or occurring(noun.state)

  Synonym:
  impossibleness
  Antonym:
  possibility, possibleness
  

  source: wordnet30
• an alternative that is not available(noun.cognition)

  Synonym:
  impossible action
  

  source: wordnet30
• The quality of being impossible; impracticability.(noun)

  source: webster1913