Qi qì · 气

"Vital energy" is a misleading translation. In TCM theory Qi is closer to functional activity — the capacity to transform, move, warm, hold and protect. It is not a substance measured like a vitamin, and no physical correlate has been identified.

Classical texts distinguish functional types: Yuan Qi (source), Zong Qi (gathering, tied to breath and the heart), Ying Qi (nutritive, moving with Blood) and Wei Qi (defensive, at the surface). Read it as a working model, not a measurable thing.