Qi is the Chinese word for Energy. There are many ways to describe Qi. Some say the easiest way to describe what is Qi is to distinguish between life and death. Where there is life, there is Qi. In death, Qi is no longer present.
Using this simple description, one can ascertain that Qi is the Life Force Energy that we all have.
From a scientific approach, one can describe Qi as a type of bioelectrical force. For example; when looking at respiration within living cells, the general equation is Glucose + Oxygen yields Carbon dioxide + Water (with by products of heat and energy.) It is in these by products of energy that we can define Qi.