Contributed by Nathan Mandigo
The simplest way to describe Qi is that it is energy.
In western physics we do not have a term that really means generic non specified energy. Even the word energy defines as something capable of doing work in some fashion, whether that is producing light, heat, motion, etc. And again, light, heat, motion, sound, electricity, etc, are all specific forms for something which we do not directly label.
Qi is undifferentiated, non specific, all inclusive. Qi is the basis for everything in the universe. When Qi does something or becomes something it will acquire a specific label, until then it justs is, it is the potential for everything.
When Qi becomes differentiated, the first aspect is density. Words like rarefied and massive describe two different extremes of Qi density. For example light is rarefied, rocks are massive. With any observable phenomenon it is the density of the Qi involved that allows us to know there is a difference. This means that at the root, the primary difference between a table and the air around it or electricity and the wire it flows through is the density of the Qi in each place.
Once Qi is differentiated by density, it is defined by its location and function. A tree has different Qi than a chair made out of wood. Water that is falling over a water wheel has different Qi than water sitting in a pond. In the body, Qi is associated with the organs and the functions performed by those organs. For example the Spleen takes the Qi from the food we eat and creates Gu Qi (sounds like Gucci) and sends it to the Lungs to be mixed with Qi from the air we breathe, Ta Qi, and creates Nutritive Qi which is sent throughout the body to nourish the body system.
Once Qi acquires a name based on location or function it can be worked with. The basis of Traditional Chinese Medicine is the manipulation of the Qi of the body to bring it into harmonious relationships and bring about health. This can be done through food, herbal formulas, bodywork, acupuncture, qi gong exercises, etc.