noun (n)

• something that baffles understanding and cannot be explained(noun.cognition)

  Synonym:
  closed book, mystery, secret
  

  source: wordnet30
• a difficult problem(noun.communication)

  Synonym:
  brain-teaser, conundrum, riddle
  

  source: wordnet30
• A dark, obscure, or inexplicable saying; a riddle; a statement, the hidden meaning of which is to be discovered or guessed.(noun)

  source: webster1913