noun (n)

• a person who habitually pretends to be something he is not(noun.person)

  Synonym:
  poseur
  

  source: wordnet30
• a person who poses for a photographer or painter or sculptor(noun.person)

  Synonym:
  model
  

  source: wordnet30
• a particularly difficult or baffling question or problem(noun.communication)

  Synonym:
  sticker, stumper, toughie
  

  source: wordnet30
• One who, or that which, puzzles; a difficult or inexplicable question or fact.(noun)

  source: webster1913