Ninjutsu is not a martial art, it is part of a martial tradition. Jutsu indicates skillset, in this case, the skills related to Nin. Nin can be translated many ways, commonly as stealth, invisibility, perseverance, and intention. Taijutsu means 'body skills', and the martial art being referred to as Ninjutsu so often is called Ninpo Taijutsu. Essentially what Ninjutsu consisted of in history were the skills of espionage, intelligence gathering, escape and evasion, and guerrilla warfare. Psychological tactics and camoflouge were certainly employed for fighting, but the Shinobi most of all wished to avoid open combat. Their way was to blend in, to escape attention, to be 'invisible' at least in their intent. Today, Ninjutsu is alive and well and grown immeasurably in degree of complexity. Any and everything one might gain skill in, that may enhance your capability to escape danger, can be considered Ninjutsu, if associated with the study of Ninpo.