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

The Free On-line Dictionary of Computing (17 December 2009):

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)


U.S. Gazetteer (1990):

Enigma, GA (town, FIPS 27596)
  Location: 31.40905 N, 83.33016 W
  Population (1990): 611 (266 housing units)
  Area: 8.5 sq km (land), 0.1 sq km (water)
  Zip code(s): 31749

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "enigma":
   Chinese puzzle, Gordian knot, arcanum, baffle, bafflement,
   baffling problem, bewilderment, bother, brain twister, cabala,
   classified information, confidence, confidential communication,
   confoundment, confusion, conundrum, crossword puzzle, crux,
   dark horse, dilemma, discomposure, disconcert, disconcertedness,
   disconcertion, disconcertment, disturbance, embarrassment,
   enigmatic question, esoterica, fix, floorer, frontier,
   frontiers of knowledge, guarded secret, hermetics, jam,
   jigsaw puzzle, knot, knotty point, matter of ignorance,
   mind-boggler, mystery, mystery of mysteries, mystification, n,
   node, nodus, nonplus, nut to crack, oxymoron, paradox,
   perplexed question, perplexity, personal matter, perturbation,
   pickle, plight, pons asinorum, poser, pother, predicament,
   private matter, privileged communication, privity, problem,
   profound secret, puzzle, puzzlement, puzzler, quandary, question,
   question mark, restricted information, riddle, scrape, sealed book,
   secret, sixty-four dollar question, stew, sticker, stumper, teaser,
   terra incognita, the incalculable, the occult, the strange,
   the unfamiliar, the unknowable, the unknown, tough proposition,
   unassuredness, unexplored ground, unexplored territory,
   unknown quantity, upset, vexed question, why, x, z