Ninjutsu (忍術?) Can sometimes be replaced with the word being ninpō (忍 法?) Is a martial arts, strategy, and tactics on the battlefield and guerrilla conducted by the shinobi (also called outside Japan as Ninja). Ninja Women called Kunoichi. Currently there are some modern styles of this martial art, according to Koryu.com, not all variations are related to the history of ninjutsu in Japan which was formerly called koryū.
The Main Letter nin (忍?) Consists of two characters. The upper character of yaiba (刃?) Means "the edge of the sword", and the lower character kokoro (心?) Means "soul" or "heart". Kanji 刀 means "sword" or "blade", Kanji 刃 means "edge of sword." If the two kanji are combined it means "infiltrating", "secrecy", "endurance", and "persistence". Jutsu (術?) Means "art" or "technique". Pō (法?) Means "knowledge", "principle" when combined with affix "nin" has the meaning of ninja art, higher than ninjutsu. The current view is that ninjutsu is only about secrecy and infiltration. However, practitioners from the field of ninjutsu said that ninjutsu is necessary for us to gain resilience to face the hard life.
There is a common misunderstanding who thinks that ninjutsu is a combination or mixture of some Japanese martial arts, eg karate mixed with aikido, judo, kendo and others. This thinking is wrong, because ninjutsu itself existed long before karate entered from Okinawa to Japan and there are also long before Judo and Aikido were created.