Qi or ch'i (氣, Pinyin "qi4"), is defined as "spiritual energy" that is allegedly part of everything that exists. Also known as Ki (Japanese), references to this sort of "metaphysical" energy is used in certain belief systems, primarily in Asia.
The philosphical origins of qi stem from the earliest times in Chinese thinking. One of the most important figures in Chinese culture is Huang Di or the Yellow Emperor. He collected and formalized what became Traditional Chinese Medicine.
Derived from Traditional Chinese Medicine, these systems assert that the body has natural patterns of this energy associated with it that flows throughout the body in channels called Meridians. Illness is the product of disrupted energy movement. Traditional Chinese Medicine attempts to correct physical maladies by balancing the flow of qi in the body using various techniques. Some of these techniques include herbal medicines, special diets, and acupuncture (which uses tiny metal spines inserted into the skin to reroute qi flow) among others.
Traditional Asian martial arts theories also discuss qi. For instance, Internal Systems attempt to cultivate and direct this energy during combat as well as to ensure proper health. Many other martial arts include some concept of qi in their philosphy.
Modern scientific and medical efforts have not demonstrated the existence of qi. Many modern researchers believe that other mechanisms may explain demonstrated results from acupuncture or other practices.