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

WordNet (r) 3.0 (2006):

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, enigma, secret,
         closed book]
    2: a difficult problem [syn: riddle, conundrum, enigma,
       brain-teaser]

The Free On-line Dictionary of Computing (19 January 2023):

Enigma

    The electro-mechanical cipher
   engine used by the Germans in World War II to encrypt and
   decrypt field orders.  Many of their messages were deciphered
   at Bletchley Park, by Alan Turing and others.

   See also: Tunny Emulator.

   (2012-03-25)