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).