I wrote a blog last year about what aiki was from my understanding of the concept at that time. In hindsight I am embarrassed by this blog and my own ignorance and have considered deleting it; however, I've decided to leave it on my blog page as evidence of my ongoing study and understanding of aiki. I now approach the subject with far more humility.
For fear of appearing foolish yet again I am merely going to limit this blog to identifying the fundamental principles that from my study of Daito-Ryu Aiki-Jujutsu and Aikido are central to the application and nature of aiki. These are:
1. Ma'ai - the appropriate distance between you and your opponent to create the maximal effect of the technique.
2. Metsuke - the discernment of your opponent's actions and movements through the focus/fixation of your eyes.
3. Kuzushi - the breaking of your opponent's balance and posture
4. Kokyuho - the practise of breathing and the timing of your breathing while executing the techniques to create the maximal effect.
5. Marui (Ju) - the circle. The geometric shape of the circle is of intrinsic importance to both Jujutsu and Aikido on a technical and philosophical level. I have read that it comes from the Japanese word "Ju" meaning gentleness or softness. It is a principle I have spoken of before when I have explained my interpretation of "Ju" as being "to yield".
6. Irimi - the point of entering into your opponent's attack. Irimi is - as far as I currently understand it - the combination of principles 1-5: it requires a good judgement of ma'ai, which is adjusted appropriately through metsuke, and the exercise of correct kokyu to effect kuzushi so that your opponent cannot finish their attack resulting in blending with that energy in order to create Ju.
I am sure there are many more principles to the successful application and understanding of aiki that I am not currently aware of, so my list is not exhaustive. These 6 principles are what I am currently trying to learn and apply adeptly. What I am learning and growing to appreciate all the time is the very martial nature of aiki. As Katsuyuki Kondo, doshu of Daito-Ryu Aiki-Jujutsu explains without learning how to apply aiki, just learning the Jujutsu techniques will be for nothing.