Ki is a force permeating all life and can be thought of as life force or spiritual energy. It can also be spelled qi or ch'i, and it's nature is commonly disputed in Chinese philosophy. Several of those who frequent The House are skilled in the uses of ki.
Thanks to the pioneering research of Son Gohan, the use of ki was quantified scientifically, allowing any person to read about it and master its use without training with some hermit, in order to make a worlkd that can defend itself. This is the reason why thousands of people have suddenly learned Z-fighter grade martial arts (in Dragonball Online and Sub. Sen); he made that knowledge accessible.