Enigma

Part of speech: noun (n.)

1. A dark, obscure, or inexplicable saying; a riddle; a statement, the hidden meaning of which is to be discovered or guessed. A custom was among the ancients of proposing an enigma at festivals. Pope.

2. An action, mode of action, or thing, which cannot be satisfactorily explained; a puzzle; as, his conduct is an enigma.