First off, I am not an authority on Ki. I have written about it, but that does not make me an expert - http://blog.aikidojournal.com/2012/0...es/#more-20054
What I can say is that in my own experience, for what it's worth, understanding Ki is as difficult or as easy as you want to make it, that there is no 'catch all' definition for it and that it is virtually impossible to separate the 'use of it' from the 'user'. Physical people have physical Ki, spiritual people have spiritual Ki, happy people have happy Ki and depressed people have a tough time. Sceptical people have...let's hear it.
People are capable of change: a physical person can learn to relax, a relaxed person can build up their muscles and a depressed person can even learn to become happy in time. The Ki that each of us has, at any given moment in time, is a reflection of our existential condition. Ki is not static, and it is not an object in some imaginary martial arsenal.
Ki is a coin of two sides, mind and body. Sure we can now measure alpha, beta, delta and theta waves that reflect back how happy, depressed or tranquil we are and there is a clear correlation between physical states and mind states, but so what? This is an intellectual approach, and all that stuff is 'about' Ki - it is just feedback, not the thing in itself.
And this is IMO points to the central problem with Ki and many other concepts that come from Chinese culture: as long as we think about Ki as an object then we will have difficulties. Without reference to yin and yang - the unification of opposites - Ki does not make much sense. But, at the same time, you don't have to be a Taoist scholar to understand Ki.
Having spent over two decades (but no longer) practicing Ki Aikido, and never learning anything about yin and yang, I now understand what was missing. To some extent the principles are built into the practice (mind body coordination, weight underside, etc) but the full value and significance is understated and not developed. But this is not only in Ki Aikido, it is true of Aikikai Aikido as well.
While some individuals have managed to 'catch on' to something as a result of their own efforts, more by accident than design, many have not and continue to practice until they hit a ceiling and can't improve. Most people in Aikido cannot get better than their teachers, and their teachers cannot improve much because they have no one to help them move forward - apart from their students who are often treated as lab rats.
But to get back to the point. Ki is not an object, and if we are able to unify mind and body (yin and yang) and access whole body power (rather than isolated muscle groups) and therefore get closer to what 'aiki' is all about, we will understand Ki and perhaps understand too that we are as likely to see ourselves 'mind bodying', as we are able to taste our tongue tasting, or see our eyes seeing or hear our ears hearing. There is always bio-feedback, but I prefer to cut out the middle man. For those looking for the absolutely definitive in a relative world, good luck.