QI is one of the most complex and multifaceted terms in all of Chinese philosophy, religion, and science. No single word can translate it adequately. Its root meaning is "moist emanation." Steam, clouds, and mist are qi, and the word appears frequently in compounds that refer to meteorological phenomena. Another basic meaning is "breath." Later, these meanings were sometimes amalgamated; the Daoist philosopher Zhuangzi (fourth century BCE) wrote, "When the Great Clod [the Earth] exhales breath, it is called wind."

During the Warring States period (481–221 BCE), the classical age of Chinese philosophy, the word qi began to be employed in an expanded variety of meanings. The concept of breath gave rise to the meaning "vital spirit," that is, the life force of all creatures. "Nourishing the vital spirit" (yang qi) by means of diet, yogic exercises, breath control, or sexual yoga became an important part of the Daoist...