"The study of the spirit" is not a functional definition. If you tell a friend you have aikido tomorrow, and someone asks, "What's that?" you're not going to tell them it's the study of the spirit, because that could mean anything and he's going to have no idea what you're talking about. The word aikido has to mean something more specific than that, or else it has no meaning at all.
Matt, you're looking for a one size fits all meaning for a process that individualizes it's meaning for each practitioner. The meaning of Aikido grows and changes with time. At the point you are at in your practice it sounds like you need Aikido to be defined in absolute terms that you can rely on to be invariant; to provide a firm footing upon which you can build your knowledge.
I think that this is very common among Aikido students. I know that it was for me. Anyway, find what works for you
and run with it. The expanded meaning of Aikido reveals itself over time according to the needs of the student.