A or was a covert agent or mercenary in feudal Japan. The functions of the ninja included: espionage, sabotage, infiltration, assassination and guerrilla warfare. Their covert methods of waging irregular warfare were deemed "dishonorable" and "beneath" the samurai-caste, 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 in the 12th century (Heian or early Kamakura era).

.ninja is an active generic top-level domain (gTLD) that was proposed in ICANN's New gTLD Program. The Registry and manager of the TLD is Rightside/Demand Media (United TLD Holdco Ltd.). The proposed application succeeded and was delegated to the Root Zone on 29 December, 2013.