The term 'ninja' is a Japanese term for a type of WARRIOR who specialized in espionage and assassination. The crossword solutions for the term 'ninja' are 'WARRIOR' (7 letters) and 'sensai' (6 letters). 'WARRIOR' is an appropriate description for a ninja, as they were trained in martial arts and combat. 'Sensai' is a Japanese term for a teacher or master, which is also fitting for a ninja, as they were highly trained in their craft.