A ninja (known in Japan as shinobi) was a lethal agent of war during the feudal period in Japan. Known for their stealth and unorthodox tools of warfare, ninjas are renowned figures in history and in pop culture for their skills in sabotage, infiltration, and assassination. Those who train in the martial art of ninjutsu are modern-day ninjas.
A ninja or shinobi was a covert agent or mercenary in feudal Japan who specialized in unorthodox warfare. The functions of the ninja included espionage, sabotage, infiltration, and assassination, and open combat in certain situations. Their covert methods of waging war contrasted the ninja with the samurai, who observed strict rules about honor and combat. The shinobi proper, a specially trained group of spies and mercenaries, appeared in the Sengoku or 'warring states' period, in the 15th century, but antecedents may have existed in the 14th century, and possibly even in the 12th century (Heian or early Kamakura era).
Contributions by 18.104.22.168, Revth, and DopefishJustin.