The Collaborative International Dictionary of English v.0.48:

enigma \e*nig"ma\ ([-e]*n[i^]g"m[.a]), n.; pl. enigmas
   ([-e]*n[i^]g"m[.a]z). [L. aenigma, Gr. a'i`nigma, fr.
   a'ini`ssesqai to speak darkly, fr. a'i^nos tale, fable.]
   1. A dark, obscure, or inexplicable saying; a riddle; a
      statement, the hidden meaning of which is to be discovered
      or guessed.
      [1913 Webster]

            A custom was among the ancients of proposing an
            enigma at festivals.                  --Pope.
      [1913 Webster]

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