Kjartan Clausen on ki

Aikido makes extensive use of the concept of ki. Aikido is one of the more spiritual martial arts and has been referred to as "moving zen." The name Aikido can be translated as "the way of harmony of ki." Exactly what ki 'is' is a somewhat controversial issue.

Some believe that the physical entity ki simply does not exist. Instead, the spirit, the intention, the bio-physico-psychological coordination through relaxation and awareness are concepts being used in the teaching. These aikidoka sometimes tend to frown upon the philosophical/spiritual aspect of ki.

Other aikidoka believe that ki does exist as a physical entity and can be transmitted through space. They, on the other hand, make use of concepts such as ki of the universe, extending ki, etc.

The fact of the matter is that there is a large portion of aikidoka who are still, and no doubt will continue be, on their "quest for ki."

Without doubt, this has been the most difficult question to write any kind of reasonably fair answer to. On the subject of the nature of ki, perhaps more than in any other area of Aikido, the aikidoka must find his or her own answer, whatever that may be. The last word on this subject will be left to the Doshu, Kisshomaru Ueshiba, the son of O-Sensei who writes in "The Spirit 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 rom the on-line aikido FAQ.