It is my understanding that O Sensei's use of the term 'Aiki' stemmed from his Omoto Kyo experiences; i.e. a mystical concept that suggests some kind of union or coming together of oneself with the universe. It is interesting to attempt to define 'Aiki' from a martial viewpoint, but it is also worth considering that the Founder himself might have defined it primarily from a religious or spiritual viewpoint.
That would be at odds; not only with his many quotes about martial use- including many comments about his atemi being deadly- but also from his actions; including going to judo dojos and dislocating someone's hip with an atemi, and his training of military people-including ultra right wing types. Were his message meant to be purely spiritual he would never have lived his entire life in...martial arts! You would also never have known his name in the first place. It was his martial prowess that gave him fame, and not his spiritual beliefs, which none of his students ever claimed to truly understand.
I think all the spiritual jargon was greatly misunderstood and among some people in Aikido it is become something of a new age, animistic, add your own quasy-spiritual belief, add-on at your pleasure, type deal that it never really was meant to be in the first place.