The study of ki is a critical component of aikido, and its study defies categorization as either "physical" or "mental" training, as it encompasses both. The original kanji for ki was 氣, and is a symbolic representation of a lid covering a pot full of rice; the "nourishing vapors" contained within are ki.
The character for ki is used in everyday Japanese terms, such as "health" (元気, genki?), or "shyness" (内気, uchiki?). Ki is most often understood as unified physical and mental intention, however in traditional martial arts it is often discussed as "life energy". Gōzō Shioda's Yoshinkan Aikido, considered one of the "hard styles," largely follows Ueshiba's teachings from before World War II, and surmises that the secret to ki lies in timing and the application of the whole body's strength to a single point. In later years, Ueshiba's application of ki in aikido took on a softer, more gentle feel. This was his Takemusu Aiki and many of his later students teach about ki from this perspective. Koichi Tohei's Ki Society centers almost exclusively around the study of the empirical (albeit subjective) experience of ki with students ranked separately in aikido techniques and ki development.
Read more about this topic: Aikido