'Qi', or more accurately 氣 (traditional) or 气 (simplified) dates way back to very early Chinese history and is not exclusively used in the medical or health fields.
In later and current periods of Chinese history it's usually considered to mean air or breath, but also vital energy or life force, this latter sense being the sense in which it is deployed for traditional medicinal techniques such as 'Qigong' - 氣功 （traditional) or 气功 (simplified) - which is literally '[getting positive results from] [the energy]'. However, not many people are aware that initially it was used to refer to 'sickening vapors that rise from a corpse' (an early concept for bacterial/airborne illness, a concept later repurposed in premodern Chinese medicinal thinking to describe the acquisition of malaria and similar illnesses in tropical areas).
Qi today is basically a catch-all word for energy flowing (possibly 'in a manner roughly like vapor') and could be reasonably roughly seen, from a medical perspective, to refer to the combined effect of circulatory, lymphatic and respiratory systems.
Being such a vague term, approaching it from a western perspective seems difficult. You would perhaps get further looking for western studies of the health benefits of Qigong practice.