To know Qigong, we must first know qi.
Qi, pronounced "chee", has no Western equivalent and therefore no precise English definition. It is most often translated as "energy", but it can also be translated as "vital force" or "life force". It can also imply breath or air. As it is a word unique to another language, I personally believe we should use qi without translating or transliterating it.
Qi is that which initiates change. To better understand this, one must consider qi as a process rather than an object.
Qi is both transformation itself and the agent that initiates transformation. For this reason, qi can be understood as a force of nature and is most often translated as "energy", as mentioned above. It may be a unique force at the metaphysical level: perhaps part of the long-sought Unified Field Theory. However, qi is not just one particular force, but rather any force responsible for initiating change.
Qi is the agent that initiates change; it is not separate from that which is changing. As such, it has a relative nature. Qi can be the relationship of space to time. It can be the relationship of water and soil, or between hot and cold air masses. Qi can be the relationship between the mind and body, or the organs within the body; between the bodily systems and the cells and tissues they serve; between the self and the forces of nature.
On the quantum level, qi is the relationship between subject and object, between oneself and what one observes. Rather than a distinct substance that stands alone, it is woven into the paradigm of observer and observed.
Qigong is the theory of qi made manifest in practice.
Qigong (also spelled Chi Kung, pronounced "chee gong" and "chee gung") is a system of exercises designed to increase interconnection in the body. This includes connections between mind, body, and breath; between different parts of the body to one another; and between different levels of the mind.
Qigong exercises work to erode tension, thereby improving the responses among different body parts and systems. They combine movement, body alignment, breath control, focus, and visualization. Qigong is referred to in the ancient texts as Tao Yin, which means "A Way of Guiding and Leading". Tao Yin was first mentioned in the Nan Jing, a third-century medical text. Our modern term comprises two words: Qi and Gong. Gong means "work"; Qigong translates to "working with qi".