Ki is an intergalactic energy panacea and also about breathing and nature and stuff. Furthermore it is solo training like yoga only done the right way. It was made up by the Chinese but the Peking Opera cleverly disguised its provenance by hiding it inside rubber weapons, calisthenics, and street magic. Lastly it is what Jesus knew about and definitely completes religions.
Pretty good summary of several thousand aikiweb posts on the subject of "ki".