Mushin literally means mind, no mind or clear / empty mind. It is a state where you are acting but not having to think about your actions. Achieving the state of Mushin is at the core of Aikido and Martial Arts and can only be achieved by years of practice.
Mushin is a state of mind when your brain is not preoccupied with anything else than the specific activity at a time, but importantly is acting on it in an unconscious state of action. Mushin is achieved when a person’s mind is free from thoughts such as fear, anger, or every other emotion in everyday life. This causes the person to be free to act and react, with no hesitation or disturbance in the activity by just following their instincts without thinking about their next move.
I can easily compare it to when you first learn to drive a car, you have to consciously think about everything you do, you consciously think about pressing pedals, turning a wheel, changing gears, looking in mirrors, stopping distances as well as dealing with all this while driving at speed with pedestrians and other vehicles around you in an ever changing environment. But once you have mastered driving your car, you do all of those things without having to think about doing them and drive back and forth everyday putting no thought into your actions and what is happening around you. That is Mushin.
When you go into Mushin, your mind is quiet, but your body is acting. This Mushin state is what you train to perfect in your Aikido allowing your spirit to guide your body through constant training and spontaneously allow techniques to occur without thought, manipulating the energy of your partner with no thoughts of, I should do Irimi or Ura/Tenkan movements or this hand or foot should go here to do a Shionage, Ikkyo etc. Train in Aikido to achieve a Mushin state.
“Cast off limiting thoughts and return to true emptiness. Stand in the midst of the great void. This is the secret of the Way of a Warrior"