ensō (円相) is a japanese word meaning "circle" and a concept strongly associated with zen. ensō is one of the most common subjects of japanese calligraphy even though it is a symbol and not a character. It symbolizes absolute enlightenment, strength, elegance, the universe, and the void; it can also symbolize the japanese aesthetic itself.
"But mystics have never been very interested in theology. Mystics are interested in direct experience, they are empirical in their approach. And the taoists, being mystics, were the only great group of ancient Chinese people who seriously studied nature. They were interested in its principles from the beginning, and their books are full of analogies between the taoist way of life and the behaviour of natural forces seen in water, wind, or plants and rocks." –Alan Watts