enigma

/ɪˈnɪɡmə/
noun
  1. A person, thing, or situation that is mysterious, puzzling, or difficult to understand.
    • The old mansion was an enigma, with hidden rooms and strange sounds at night.
    • Scientists are still trying to solve the enigma of how the ancient pyramids were built.
    • Her sudden decision to quit her job and travel the world remains an enigma to her friends.
  2. A riddle or puzzle, especially one with a hidden meaning.
    • The poet's work is full of enigmas that readers love to decode.
    • In the game, each level presents a new enigma that requires logic to solve.
    • The Sphinx's enigma challenged all travelers who passed by.