Source : Webster's Revised Unabridged Dictionary (1913)

Enigma \E*nig"ma\, n.; pl. {Enigmas}. [L. aenigma, Gr. ?, fr. ?
   to speak darkly, fr. ? tale, fable.]
   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.

Source : WordNet®

enigma
     n 1: something that baffles understanding and cannot be
          explained; "how it got out is a mystery"; "it remains
          one of nature's secrets" [syn: {mystery}, {secret}, {closed
          book}]
     2: a difficult problem [syn: {riddle}, {conundrum}, {brain-teaser}]

Source : Free On-Line Dictionary of Computing

Enigma
     
         The electro-mechanical {cipher}
        engine used by the Germans in World War II.  Many of their
        messages were deciphered at {Bletchley Park}, by {Alan Turing}
        and others.
     
        (2000-09-30)