What, actually, is "Aiki"?
William Gleason: Aiki is the unified state of Heaven, Man, and Earth. It is the vertical or spiritual unification of an individual and his or her environment. This state is absolute and therefore has no opposite or back side. When it is brought into motion it becomes ki-ai or yamabiko, the mountain echo, as O-sensei called it. This means that, maintaining your vertical unity, your ki goes out to your partner and again returns to yourself.
for the record; i do not understand that answer in at least six different ways.