Ki is the Japanese rendition of the Chinese term Chi, which is, in a drastically simplified explanation, the spiritual energy that underlies/infuses/makes up all things in the cosmos. Plants, animals, places, elements, people, all possess their own internal ki energy, which can be manipulated and harnessed by those who have undertaken the correct regimes of cultivating higher physical, mental, and spiritual strength and awareness. At the lower end of the spectrum, manipulating ki can allow one to exert greater control of the body, resisting pain, moving with unnatural speed, striking with great strength, and so forth. Higher ki manipulation is often indistinguishable from magic, allowing for superhuman physical feats like walking on clouds or water, flying for miles by riding on a thrown sword, going for years without food or sleep, or even blatant acts of elementalism such as conjuring flames and controlling water or earth.
Dungeons & Dragons 4th Edition initially considered using a Ki power source, but ultimately decided that it was superfluous, as they could not find any justification that truly distinguished it from "The Martial Power Source, But Asian". As a result, the Monk was folded into the Psionics power source, as Psionics actually taps into a similar vein of Asiatic and Indian mysticism as Ki.