(sigh) once again I recommend The Expressiveness of the Body and the Divergence of Greek and Chinese Medicine
, a very readable book by Shigehisa Kuriyama on the theoretical bases of classical medicine and their distinct versions of what comprises a human being.
Coming to aikido w/ many years familiarity w/ TCM I had no problem with the concept of ki, using ki, moving ki, etc.
I agree that the term has no meaning to many folks, or varied meanings. That's why I tend to not use it when I'm leading a class. I prefer to use the words "energy" and "intent" depending on the situation - even though they are not the same thing as "ki", in terms of visualization/somatization/application by an aikido student they are close enough to be practical. My two cents.