Ki is the Japanese on'yomi formatting of the Chinese character Qi/Chi (気 or 氣, pronounced as "chee"), and is one of the five elemental categories of magic. It is the oldest among all magical elements and is also considered the foundation of all magic. It has the ability to amplify the strength and effects of the other four magics. It is also the least understood among scholars of Magical Theory of Science.
According to the reports of the P.W.A.B, most of the people who control Ki appear to be of Eastern descent; therefore, the ability to use Ki is thought to have some connection to the genetic makeup of Eastern peoples. Lifestyle and culture may also have an effect on the affinity to Ki as the Japanese, being very spiritual people, have a natural talent of using it. Because of this potential, they were destroyed by Justice.
Notable Ki users include Jam Kuradoberi, Chipp Zanuff and Kliff Undersn despite the latter two being from America and Switzerland, respectively. Japanese like Anji Mito, Baiken and May had also displayed several Ki-powered attacks. Mizuha from the manga is also a user of Ki.
In real-life, qi/ki is considered by Asian philosophy to be a type of life force or energy flow that literally often pertains to the atmosphere around oneself. It is usually is the basis for most forms of Asian medicinal practices and Chinese martial arts; the art of practicing qi/ki is known as qigong/kikou (気功).