This is a question that can't be answered so easily. In fact, on many levels, it can't be answered at all. Ask a ten aikidoka this question and you will probably get ten different answers.
Firstly, aikido is a martial art. The art focuses not on attacking an opponent, but rather on using the attacker's own energy to gain control of him and neutralize his attacks.
Not a fighting system, but a way of the warrior. Aikido is a true martial way that evolved in the historic tradition of the Japanese samurai.
The second dōshu, Kisshomaru Ueshiba, had this to say, "[Ōsensei] based Aikido upon circular movements to redirect aggressive and offensive attacks back to the assailant.
"Let it be clear, however, that ... Aikido is a refinement of traditional martial techniques combined with an exalted philosophy of the spirit. It is a method of forging mind and body."
OK, so we can assume it is a martial art or way, but what was that bit about exalted philosophy of the spirit"?
The United States Aikido Federation website translates aikido as "the way of unity with the fundamental force of the universe."
Ōsensei also had a strong spiritual drive, and brooded over the futility of a path based on victory over others. Studied in earnest, budo is more than a science of tactics and self-defense -- it is a discipline for perfecting the spirit.
"The secret of Aikido," he wrote, "is to harmonize with the movement of the universe and bring ourselves into accord with the universe itself." O-Sensei maintained that budo is a work of love, a path to overcome discord in ourselves and bring peace to the world, "to make the heart of the universe one's own heart."
But how can a martial art embrace such seemingly contradictory concepts?
Any serious study of Eastern philosophies by Westerners immediately runs into fundamental differing view of reality.
In the West, we were educated in the spirit of the scientific method, logic and industry. We tend to believe that all questions can be reduced to answers by reason alone.
However, Asian spiritual ideas tend to fly in the face of such a view. Buddhism posits that a "person" can be described as both something that exists and does not exist and neither exists or does not exist. The main point being that any word or concept cannot really describe what is essentially indescribable. The duality of reality is considered an illusion, but the non-duality of reality would be an oversimplification.
That's why Osensei said things in such a cryptic fashion. He was so far ahead of us, yet he maintained we can "catch the secret" and do what he did in 3 months. Since we all don't do that, we must be doing something different.
Henry Kono Sensei calls it an "unseeable matrix we can’t comprehend."
Aikido is an original and unique art. It is concerned with “the philosophy of the creation of all beings in the universe”, “the rationality of circular movements” and “the principle of kokyu power.”
Well, I'm trying to catch that secret. I hope you are too.