When I formulated my definition of Ki, I based it on the three harmonies model, "mind leads intent, intent leads Ki, Ki leads power", which is one of the bases of how I like to train. I tried, however, to place the definition closer to the realm of physical reality. Since "spiritual energy" is not a defined type of energy nor can we measure its sate, I used "physical energy" (resulting from our tangible body mass), which can be transformed into either mechanical and/or thermal energy, and finally expressed as power (energy/time), the process being initiated by the mind or as some call it "intent" (which follows a desire).
That's the kind of informative post that drives the discussion forward. My own working definition of Ki as a manifestation of a coordinated mind and body employs spirit as a quality I possess which can enhance or retard my ability to integrate mind and body. As such, spirit plays a vital role in my training, but not as a form of energy or a mystical connection to some greater truth. Training employs exercises and waza that are designed to facilitate the smooth unification of mind and body in order to manifest Ki. The fact that theses exercises and waza are built around martial techniques makes the whole experience doubly rewarding.