The term 'Teenage MUTANT Ninja Turtles' is a popular franchise that originated in the 1980s. It is based on four anthropomorphic turtles named after Renaissance Italian artists, who were trained by their rat sensei in the art of ninjutsu. The word 'MUTANT' is used to describe the turtles' special abilities, which were granted to them by exposure to radioactive ooze. The six-letter solution to this crossword is 'MUTANT'.