Ninjitsu (properly, "Ninjutsu"), sometimes called ninpō, is a martial art, strategy, and tactic of unconventional warfare and guerrilla warfare as well as the art of espionage purportedly practiced by the ninjas.
In Mortal Kombat
It is used as primary fighting style by Cyrax in Mortal Kombat: Deadly Alliance and Armageddon, and by Sektor in Mortal Kombat: Tournament Edition, because of him being a palette swap of Cyrax in that game.
In Mortal Kombat X, one of Scorpion's character variations is Ninjutsu. This variation focuses on utilizing Scorpion's ninjitsu training over his demonic powers, which involves the use of his dual swords.
Ninjitsu is one of the only fighting styles made up of several other fighting styles, like hand to hand combat and illusionary combat.