a puzzling or inexplicable occurrence or situation

His disappearance is an enigma that has given rise to much speculation.

a person of puzzling or contradictory character

To me he has always been an enigma, one minute completely insensitive, the next moved to tears.

a saying, question, picture, etc., containing a hidden meaning; riddle.

( initial capital letter ) a German-built enciphering machine developed for commercial use in the early 1920s and later adapted and appropriated by German and other Axis powers for military use through World War II.

noun

bewilderment , cliffhanger , conundrum , crux , cryptogram , gordian knot , grabber , knot * , mind-boggler , mind-twister , mystification , parable , perplexity , problem , puzzle , puzzlement , puzzler , question , question mark , riddle , secret , sixty-four dollar question , sphinx , sticker , stickler , stumper , teaser , tough nut to crack , twister , why * , inexplicable , mystery , puzzling , rebus