The term 'Brain Twister' is used to describe a puzzle or problem that requires a great deal of thought and concentration to solve. The two crossword solutions for this term are 'ENIGMA' and 'SECRET', both of which have 6 letters. An ENIGMA is a type of puzzle or riddle that is difficult to solve, while a SECRET is something that is kept hidden from others. Both of these words can be used to describe the concept of a Brain Twister.