Qi or, as spelled in Wade-Giles, ch'i (氣 in pinyin: "qi4"), is defined as "life energy" or "spiritual energy" that is part of everything that exists. Also known as Ki (Japanese) or Gi (Korean). References to this sort of "metaphysical" energy are used in certain belief systems, primarily in Asia. A common pronunciation is as in the English "key".
The philosophical 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 needles 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 philosophy.
Many western researchers believe that other mechanisms may explain demonstrated results from acupuncture or other practices, despite the fact that Qi can be measured using modern technology. To this date, there is no proof that something other than Qi brings about the effects created by the use of Traditional Chinese Medicine.
There is active research comparing Qi to biophotons. Mainstream science considers all claims of Qi actually existing to be religious claims that have no physical reality or proof. Claims that qi has been related to any physical or biological phenomenon are regarded as pseudoscience by mainstream western science.
Currently, individuals investigating 'Qi' promote three differing perspectives regarding its qualities and processes: (1) that these energies exist but do not affect organic life in any way; (2) that subtle energies are a "fifth force," distinctly different from the other four standard forces; or (3) that the variations and complexities of subtle energies manifest the four forces and elements that compose all force and matter. This last perspective, if proven true, would indicate that 'Qi' is a particular expression of space at the sub-quark level, according to western science.
Resources: ENERGY MEDICINE: The Scientific Basis, by James L. Oschman, PhD, Churchill Livingston, 2000