Qi

In the earliest texts qi meant vapour from heated water, then breath; Roth finds it the stuff of all things only by the Huainanzi of 139 BCE.

Harold D. Roth, Original Tao: Inward Training (Nei-yeh) and the Foundations of Taoist Mysticism (Columbia University Press, 1999), ch. 2

The vital energy or life-force central to Chinese philosophy, medicine and martial arts.

Sources1 source

Scholarship

Harold D. Roth, Original Tao: Inward Training (Nei-yeh) and the Foundations of Taoist Mysticism (New York: Columbia University Press), 1999, Neiye verses I, VIII, XV; ch. 2 on ch'i; ch. 4 on the jade tablet inscriptionRoth's translation of the Neiye on qi and its refined essence, his dating of the Neiye to about the mid-fourth century BCE, and his account of qi meaning vapour in the earliest texts and universal stuff by the Huainanzi of 139 BCE.
The vital essence: it is the essence of the vital energy.