Japanese word meaning "spirit" energy believed to be the source of life. It was generally held that this powerful source of energy could be tapped only if a man had stabilized that position of inner centralization in the hara. In India this vital force has been known for centuries as prajna, in China as ch'i, and in Japan as ki.
The coordinated energy of the hara could infuse a man with tremendous vitality and make him powerful in action, more so that the man who had developed muscular power alone. As might be expected, the range of the methods of using these inner techniques is considerable, but all of them included, in addition to meditation and concentration, the fundamental exercise of abdominal breathing.
In ancient texts, the word itself is translated as air, atmosphere, breath. The doctrine was a main source of metaphysical and intellectual speculation for Indian, Chinese, Tibetan, and Japanese scholars, philosophers, and religious leaders. Finally, it was adopted by almost all the important schools of martial arts.
Ki, or ch'i, is a concept that is both natural and simple. Those who make of it something mysterious and akin to magical power miscomprehend what is essentially a common property of all human beings. Learning to release and utilize ki is where the difficulty lies. Its function in the human body has been described as "electricity that flows back and forth along the wires of our nerves," and as "the psycho-physiological power associated with blood, breath and mind; the biophysical energy generated by respiratory rhythm."
Further reading: Modem Bulutsu and Budo, Donn F. Draeger, 1974; Secrets of the Samurai, Oscar Ratti and Adele Westbrook, 1973; The Book of Ki, Koichi Tohei, 1976; Ki In Daily Life, Koichi Tohei, 1978; Kick Illustrated, July 1980.

Oriental martial arts mirror many of the beliefs and customs of the Orient. More than merely combat techiques, most martial arts offer their practitioners a way life based on Eastern philosophy. Martial arts can be best understood if the following aspects of Oriental thought are understood. The expression "martial arts" refers to numerous styles of combat originating in the Orient. The most common martial arts are aikido, judo, jujutsu, karate, kung-fu, and tee kwon do. All of these many arts share certain customs and practices with which the student should become familiar.