Synonyms/Hypernyms (Ordered by Estimated Frequency) of noun enigma

2 senses of enigma

Sense 1
mystery, enigma, secret, closed book -- (something that baffles understanding and cannot be explained; "how it got out is a mystery"; "it remains one of nature's secrets")
       => perplexity -- (trouble or confusion resulting from complexity)

Sense 2
riddle, conundrum, enigma, brain-teaser -- (a difficult problem)
       => problem -- (a question raised for consideration or solution; "our homework consisted of ten problems to solve")