Something or someone puzzling, mysterious or inexplicable. / A riddle, or a difficult problem.

(historical) A German device used during World War II to encode strategic messages.

plural of enigma

(historical) A small portable electro-mechanical rotor machine used to encrypt and decrypt secret messages, made famous by Germany in WWII.

plural of Enigma machine

(idiomatic) Something very mysterious and hidden.

Pertaining to an enigma. / Mysterious. / Defying description. / (variant) Enigmatical.

the study of puzzles

Acting in a manner that suggests an enigma. / Behaving mysteriously or strangely. / Functioning in a way that is unexplainable.

Pertaining to an enigma; enigmatic; mysterious.