Mushin is a mental state into which very highly trained artists are said to enter during practice and execution of their art. The literal translation of mushin is "no-mind" but is often translated as "unfettered mind." A mind not fixed or occupied by thought or emotion is open to everything.
According to wikipedia
"Mushin is achieved when a person feels no anger, fear or ego during combat. There is an absence of discursive thought and judgment, so the person is totally free to act and react towards an opponent without hesitation. At this point, a person relies not on what they think should be the next move, but what is felt intuitively. The mind could be said to be working at a very high speed, but with no intentions, plans or direction."
I myself think of these state as being free from conscience thought and decision making processes. We move because we must. We fit appropriately to situations with correct attitude, movement and principles. In fact I feel the previous sentence is a definition of Aikido as well.
Another interesting quote from wikipedia...
"Some masters believe that mushin is the state where a person finally understands the uselessness of techniques and becomes truly free to move. In fact, that person will no longer even consider themselves as "fighters" but merely living beings moving through space"