I was reading this thread and thinking about Gozo Shioda sensei's more down to earth definition which then turned up in another thread
In aikido we often use the word 'ki', or energy, but this word covers a variety of meanings. "Ki" as it is manifested in the performance of techniques is what we have when the components of correct posture, center line, breathing, the explosive power of focused energy, timing, etc., come together so that we reach the highest state of perfect balance. It might be said that 'ki' is the 'mastery of balance'.
To mangle another quote, sufficiently advanced technique is indistinguishable from magic.