In our teachings, you need to feel the Qi, understand its origin and exercise to master its true meaning than just read terms in common textbooks.
Even in the TCM textbooks, the Qi is defined merely as vital energy circulating in meridians system instructed in acupuncture. Actually, there are more meanings in the term.
To learn the true meaning, we need to check into the calligraphy of the original Chinese word. There are three written formats of Qi, indicating different resources and properties. 气and氣 are two common written forms for Qi in Chinese, the second one adding more element of “rice”, which indicate the Qi transformed from food. The first one is indicating the Qi from breath. Of course, inhaling and exhaling promote the vital life activity. In Taoist tradition, there is a third form, unite “emptiness” with “fire” to indicate the Qi of primary energy from space and empower life.
The above three different kinds of resources of Qi are associated with different body system in TCM. For example, the Qi in respiration system is also related with the kidney system. In terms of Qi, “yuan qi”, a namely primary energy that the new-born carried from parents is restored in the “ming men” related with right kidney system and interpreted as “life gate”. That energy helps to warm and start-up the new life development until a later age of 40, then be replaced by the individual backup after birth. If the transferring had any issue, we could see a deficiency in lung function or spinal degeneration. The “yuan qi” also impacts on the hormonal system to be related to liver and kidney system. Detailed study demands more time to cover all details, and most textbooks did not even mention those.
When the Qi flows into a different system, the name will get changed to localize the related function, such as “liver qi”, “stomach qi” etc. According to the different flow direction, it also is cataloged as upward Qi, downward flow Qi and horizontal level Qi etc. In Oriental Medicine, the same character could be described with a different name in Chinese or Indian medicine. Back to the breathing Qi, there are three levels and one of them is named “infant Qi”, referring the inner flow of Qi prior to birth and still manifest in deep sleep or energy exercise post delivery.
In TCM, the more classical or a bit sophisticated study of Qi is associated with astronomy. The specific Five Orbit and Six Qi usually introduced in the first chapter of TCM textbooks for the fundamental theory. The 1960 edition of TCM textbook still have it and later editions get them canceled by political reason and not much introduced in the later English copies. To master that universal energy regulation, students need to be familiar with lunar calendar and Chinese historical philosophy, especially understands the book of Change, ie. IChing.
It is quite an effort to learn the true meaning of Qi. The above wording only shows a little percent, as a thorough discussion will be a big book thicker than the common TCM textbook. For those who used to believe they learned about Qi, a continuing education will upgrade further understanding and there are lots of other terms like Qi in TCM awaiting more explore. Anyway, TCM has a history over 2500 years by record and thousands of volumes of documents and books carrying the valuable experiences.