... The perplexing problem is to give even a tentative translation for ki. It may be relatively easy to give a linguistic or analogical explanation. To render ki into a single foreign word is almost impossible.
If a person knows the Japanese language, we can begin to explain ki by referring to its many compounds and convey a general sense of the term. If not, we are forced to come up with a foreign equivalent. Since we lack a precise equivalent, the translation will depend on the emphasis we place on the diverse dimensions of ki. That is, we can stress the spiritual aspect (spirit, soul, ethos), the affective aspect (sense, intuition, feeling), or the psycho-physiological aspect (breathing, breath). If we understand ki as primarily spiritual, we can speak of spirit in English, esprit in French and Geist in German. If we approach it from the affective side, it would be something like feeling, or intention in English and Stimming in German. If the psycho-physiological aspect is emphasized it would be close to the Greek psyche or English ether.
On pages 25-26, the Doshu writes,
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 breathingexperienced in practicing 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 defies coverage by a single definition.
From Kisshomaru Ueshiba The Spirit of Aikido, page 30, 1984.