Perhaps I took the juice out when I suggested that we should remain civil.
My sense of aikido isn't far off what Mark Murray suggests above, but here's another take I have on it as well that extends the definition out into the philosophical realm a bit as well.
Let's define aiki as appropriate fitting. That's how I currently understand it anyway.
Aikido, to me, is about learning how to fit myself to my partner's attacks appropriately, such that I can achieve the outcome I desire.
A view, with which I disagree, often put on that interaction by many students of aikido is that the outcome must be non-violent. In my mind, the interaction itself is violence, and therefor cannot be non-violent. However, I do agree and believe that to be aikido, the outcome should result in the least possible harm to all involved, starting with myself and extending out as far as possible.
I don't believe that this means the least possible harm in that instant. I believe that this means the least possible harm for all time. This might necessitate serious injury or death in the process of doing what is necessary.
As someone who's been in street fights with and without weapons in the Middle East and the US, and held jobs as a bouncer and security at some rowdy music festivals, I think it unlikely that hurting people is always necessary, but I do not feel that it is automatically "not aikido" should it become necessary.
Each circumstance and situation dictates the appropriate action.
What we do when we train on the mat is mostly NOT aikido; it's training to learn how to DO aikido if and when the time comes.
Is (appropriate) sparring aikido? Training with (appropriate) resistance? Are ki exercises aikido? Is practicing randori to music aikido? Are solo movement exercises aikido? Are there groups of techniques that are or are not aikido?
I think we can all have legitimate feelings about these things for and against, but honestly, I find arguments against various training practices, no matter who innovated or did not innovate the idea are largely just silly because the arguments seldom explore the principles being taught by the idea and accept or discard the practice on that basis. Usually the argument is, "did O-sensei do it?" If the argument were how "this practice will instill the wrong principles" in us, then I think there is more basis to the discussion.
IMO, none of these exercises (which I took from various examples I am aware of rather than just my own practice) are aikido at all, they are merely tools to help us practice the principles of aikido if properly
practiced. To me, the principles are what makes what we do aikido, and not the specific set of techniques, exercises, or rituals. These are just the pedagogy that do make up a critical part of the character of the system or approach being taught, but are not the thing itself.
Aikido is not the forms being practiced, but the principles.
In that light, I think that all too many in the aikido world do not do anything like what I would consider aikido when on the mat. Many people train to feel good when they should be training to feel uncomfortable and stretch themselves. We train to discover or reveal the principles and to inculcate them into our instinctive responses so that we can become practitioners of aikido, hopefully able to apply those principles in our everyday life or in moments of great stress.
If we're just getting on the mat and getting a workout, than I sincerely doubt the principles are being trained or consciously practiced in those moments. A workout is great, but shugyo
to me is not simply a physically intense workout, but one that is intellectually, emotionally, and psychologically intense and has me sweating even if the training is going at glacial speeds and maybe no one is even falling down.
Many of us do not put the principles in practice in our daily lives no matter what we do on the mat. It is in putting the principles to practice that I, personally, consider to be aikido.