A ninja (敹????? or shinobi (敹????) 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).