Ki is a
concept that I have come across since I started practicing
aikido a few months ago. The impression I have of it is that it is considered to be a
substance or
energy that flows though the Universe, similar in a way to
The Force in
Star Wars (not meaning to trivialise the idea of ki, but I think this is a good
example). By performing an aikido manouevre you are extending ki and from some peoples
point of view, any defence,or for that matter any
attack where ki is extended is an aikido technique, even if you're hitting someone over the
head with a
chair.
On the subject of the nature of ki each aikidoka is left find their own answer, but here is the argument from the son of the founder of aikido
"We may hear students say that `It is a feeling of some kind of energy coming forth from mind and body in harmony.' Or `It is a strange, vital power which appears unexpectedly at times from an unknown source.' Or `It is the sense of perfect timing and matched breathing experienced in practising aikido.' Or `It is a spontaneous, unconscious movement which refreshes mind and body after a good workout,' and so forth.
Each answer is valid in the sense that it is a true reaction gained through actual personal experience. And being a direct expression of a felt condition, it contains a certitude that cannot be denied. If this is so, the differences in responses is negligible, and the great variety attests to not only the difficulty in precisely defining ki but shows that the depth and breadth of ki defy coverage by a single definition."
-- from "The Spirit of Aikido" by Kisshomaru Ueshiba, the son of O Sensei