This is an ever returning subject, of course. The nature of ki. It's not at all something unique to the Eastern traditions. I recently published a book exploring lots of synonyms to ki - such as prana, pneuma, spirit, ruach, and on and on.
Aristotle was probably right at the core of it, when he wondered what makes it possible for us to move. To the ancient mind, voluntary movement was what defined life. So, the life force was something that made us move.
As I have said many times on this forum, I like to think of ki as the ether of intention. We will to move, and the ether or agent of this will is ki.
At least, that works well in the budo applications of ki