O-Sensei himself says it in an interview. You can find it somewhere on http://www.aikidofaq.com
Sorry I don't know exactly where it is.
Yeah, I would also like to know if "Aki" is even a word. Anyway, another question, what, if any at all, is the difference between "Jutsu" and "Jitsu". Don't they both mean the "science" or "study" of?