From the Aiki News 1986 Friendship Demonstration (video for sale at aikidojournal.com):
"What is aiki? Each person has a different theory on this, but in aiki you harmonize your ki with the ki of the opponent. How?... an opponent has ki ready to attack you and you get ahead of him. This is aiki. You harmonize your ki exactly with your opponent's attacking ki. Then you proceed one more step to a rational place to defeat him. In this way the technique is applied in a flash. You perceive his attacking ki quickly and immediately deal with it in an appropriate manner. This is the way of aiki... of aikido."