Ki is a basic Japanese term that means energy, derived from the word "Genki" like in the Japanese term "Genki Dama" for the Spirit Bomb. Ki is the energy that is used to compose life force, power levels, energy aka ki blasts, flight, and most of the techniques used in Dragonball.
Ki is also a force of the real world. Many people in Asia, and all around the world follow in the belief that all living things possess ki. Humans in reality have been suggested and in some cases, shown to be able to manipulate and master the flow of ki, and use it for mainly healing practices, but also for defense/offense. Martial arts like Qi-Gong, Tai-Chi, and Kichido all emphasize and teach people to become aware of, and manipulate their ki flow. Mastery of the ki arts in reality is very rare, and in most peoples cases, would take more than one natural human life time to master.
Other terms that mean the same basic concept:
Chi - (Chinese)
Qi - (Chinese)
Kai - (Indian)