Chi also known as qi is that which gives life. In terms of the body, chi is the life force of a live human being.
To use a Biblical reference, it is that which God breathed into the dust to produce Adam. Chi is also the basis on which acupuncture is built, a system whereby chi energy blockages are removed from the meridians in the body allowing chi energy to flow again.
A strong life force makes a human being alive, alert and present in the moment whereas a weak force results in sluggishness and fatigue. You can increase and develop your chi to overcome illness, become more vibrant and enhance mental capacity.
The concept of a life force is found in most of the ancient cultures of the world. In India, it is called prana; in China, chi; in Japan, ki; for Native Americans, the Great Spirit. For all these cultures and others as well, the idea of life force is or was central to their forms of medicine and healing.
Both Traditional Chinese Medicine and acupuncture are based on balancing and enhancing chi to bring the body into a state of health.