I have to disagree with Peter on the usage of "ki" in the Japanese language. The language is full of expressions using "ki" even in isolation. One of the most common phrases is "ki wo tsukete" literally meaning "put on your ki," really meaning "be careful."
I think what everyone is discussing is a specialized usage of the word that, in my opinion, is not originally Japanese. In Japan, ki has a very general meaning, something that is hard to put into words. I think what we are talking about has much more of a Chinese influence. "Ki" in Japanese and "Chi" in Chinese have meanings which, while having some overlap, are fundamentally different.
In Chinese thinking, there is no confusion about chi. It refers to very specific energies that have different types and are clearly labelled. The confusion, as I see it, came about through Japanese martial arts teachers who were influenced by the Chinese thinking. The two ideas get mixed. For westerners, understanding what is going on when hearing the word,"ki" becomes very difficult.
What I have done, is to read as much as I can about the subject in the two cultures. However, I still have a long way to go as it is still confusing.