If one isn't experiencing ki, no definitions can help, because it is an awareness, an experience.
It simply isn't a thing but a perception, a sensation.
気 is quite a common word in Japan.
It is defined as Care, Spirit, or Mood. Spirit as in "He's in high spirits":
Ki o tsukeru; to "watch out" to stay aware.
Ki ni naru; to be bothered by something.
Kibun: mood, the way you feel at any moment.
When you have done Aikido long enough that you start feeling, "aha" ... start feeling the power in movement, start feeling rather than thinking about it... at this point ki exists.
"Who feels it knows it," and if you don't feel it it simply does not exist.
You first start to feel this sensation in the palms of your hands when you hold them in the "ten" position,when doing tenkan. There are well-known photos of Osensei, at the moment after Kokyuhou, his arms extended,
http://aikidoheyrieux.free.fr/osensei92.jpg
I believe this is the technique where one first begins to feel a curious sensation, like something very light, but substantial resting on their palms. Hold that position and weigh that feeling in your palms, as you breath. You start to feel it here.
When you feel it it becomes very real to you. And like a person who "feels it" is more powerful than a person who doesn't yet feel the flow of what he is doing, ki is the "key" to Aiki power.
Words are only words, and any definition of ki is only going to be words. Know that if you continue Aikido, you will start to perceive the power of breath, of turning, of sensitivity.
When you feel it you will know it and know that the quickest way to understand it is to continue Aikido until you feel it.