I find it rewarding to regard ki as "the ether of intention". Maybe you could also call it will-power.
Of course, this old concept has changed meaning through time. My impression is that it originally was a way of trying to explain life and movement, much like the Greek pneuma, the Hebrew ruach, and so on. A life energy, related to air and breathing - maybe simply because if stopping to breathe, you die quite quickly.
Anyway, whatever a modern explanation of ki would be, I find it a very useful concept in aikido training - well, in life
I have written about it, and how to exercise it, on my website: