For the modern practitioner of Ninjutsu, or Ninpo, the term Ninjutsu represents a set of unarmed and weapon techniques from a number of ryuha, namely Koto ryu, Gikan ryu, Gyokko ryu, and Togakure ryu, among others. The techniques include various methods of fighting, leaping, hiding, walking and running methods, as well as sword evading techniques, and special utilization of the body. Similarly, the arsenal of weapons includes a variety of conventional weapons such as Tachi and Yari, and unconventional weapons such as Shuko, Kusari fundo, and concealed weapons. In any case, the combative characteristic of Ninjutsu, be it defensive or offensive, is commonly accepted as the essence of Ninjutsu.

However, a close analysis of historical records, from as early as the eighth century to as late as the nineteenth century, show that the fundamental nature of Ninjutsu was in fact methods of infiltration into unfriendly, often hostile territory. Descriptions of such infiltration usually talk about a general who sends his agents to infiltrate his enemy's encampment, castle, or province. The purpose of that infiltration was to gather information about the enemy, to cause disorder, and to disseminate false information. Sometimes infiltration was the first act of a military confrontation, that is, an agent was sent to infiltrate a fortress in order to open its gates from the inside to allow warriors into the fortress. And sometimes the purpose of infiltrating the enemy territory was simply to assassinate the enemy's general. It is interesting to note that most descriptions of such infiltrations are only a minor theme within a larger narrative, that the term "ninjutsu" does not even appear, and that only rarely do we get a description of the method of infiltration. The most common terminology used in all of these historical records is, shinobi komu and shinobi iri, which generally mean infiltrating incognito.

The only outstanding exception to most records are those written by Iga and Koga warriors about their own methods of infiltration. Especially in the Bansenshukai, a seventeenth century multi- volume compilation, there is an explanation of methods of infiltration into a fortress or a castle, accompanied with sketches. While these Iga and Koga records include sections on special weapons, history, philosophy, astronomy, topography and more, it is clear that the essence of their activity focuses on entering an enemy territory for reasons I have mentioned earlier. The unavoidable conclusion is that Ninjutsu in essence, at least from a purely historical perspective, is the skills involved in the act of covert infiltration for military purposes. Naturally, we should now ask, what are all these fighting skill that we now call with such confidence "Ninjutsu"?

The answer to that is not given in all those historical records which I have turned to in order to understand what Ninjutsu is. In fact, there is no known pre-modern historical record that systematically describes, or at least lists the titles of fighting techniques used by those warriors who specialized in infiltration and covert activity. The only records, which I am aware of, are those handed down by a number of late Edo period specialists to Takamatsu Toshitsugu who then passed on the records and knowledge to a handful number of disciples. If there are other genealogies of Ninjutsu related ryuha they remain unknown, but it is most likely that other genealogies did not survive the transition to the modern period and that if anything remains of them it is only in the form of written records, which are hidden somewhere--perhaps without their owner's knowledge of their contents.

The final conclusion of this brief analysis is that Ninjutsu until the modern period refers to knowledge and skills for entering enemy territory and fortifications in secret or in disguise. It is a universal term that applies to groups or individuals who engaged in covert operations or infiltration regardless of regionalism, clan affiliation, or historical period. On the other hand, Ninjutsu as it has been viewed after the Second World War is a systematic collection of fighting skills according to ryuha and respective genealogies. These ryuha, contrary to the universality of the term Ninjutsu, are identified with specific groups and clans who existed in specific regions in certain periods before the modern era. However, this differentiation between fighting skills, which we now identify by the ryuha, and the clans' or individuals' covert activity, for whatever purpose it may have been, does not mean that we are all wrong in calling these fighting skills Ninjutsu. Throughout history we witness continuous processes and shifts in the characteristics and definition of things. We should therefore view Ninjutsu as having gone through a transition into the modern period, at the end of which its meaning changed. It is important, however, to keep in mind the distinction between pre-modern and modern Ninjutsu.

All info taken from ninpo.org