Chi (Qi) is the flow of energy that determines the liveliness of your body. It is this natural energy differentiating a corpse from a live person.
Qi's concept rests in many cultures. In Chinese, it is the central and most important aspect of medicine and martial arts. Referring to the Bible, Chi is the energy by which God produced Adam. Qi in India is called prana, in Japan, ki, while the native Americans call it the Great Spirit.
Flow of energy that sustains human life is a belief across the globe especially in Asia with the Chinese philosophy, ancient Hindu yoga, etc. This concept has even been taken into the Western Pop Culture - 'The Force' in Star Wars.
The energy can be enhanced in a human being. A strong force within will make a person lively and alert while the weak energy will result in sluggishness and fatigue. Chi can be developed to overcome illness and enhance mental capacity.