The term MIND-BOGGLING PUZZLE can be summarized by the crossword solution ENIGMA, which is a six letter word. An ENIGMA is a type of puzzle or riddle that is difficult to solve and requires creative thinking. It is often used as a metaphor for something that is mysterious or perplexing. The term MIND-BOGGLING PUZZLE is an apt description for this type of puzzle, as it implies that it is so difficult that it can leave one feeling baffled.