Another untranslatable term. It is sometimes rendered as energy, but Chinese thought does not distinguish between matter and energy. Chi could perhaps be thought of as lying at the border between matter and energy, but it is impossible to define the concept clearly, partly because classical Chinese thought doesn’t seem to go in for definitions much.
The ancient Chinese preferred to describe things in terms of what they do rather than what they are. Thus chi sustains all kinds of movement and change, it protects against harmful influences, it transforms food into other substances as well as into chi itself, it holds organs in place and prevents excessive fluid loss, and it warms the body. It flows in the blood vessels and also in special channels (meridians), in conjunction with the blood. (Chi is yang, blood is yin.) A fascinating feature of this scheme is that chi and blood were thought of as circulating in a pumped system; thus the ancient Chinese anticipated William Harvey’s discovery of the circulation of the blood by hundreds of years (indeed, as early as the second century BC).