The ensō is a common symbol associated with zen buddhism. It is simply a circle drawn by hand, however as it originates in the Asian calligraphic tradition, it is almost always represented as a single black brushstroke.
The ensō is given many interpretations, but it traditionally a symbol of infinity, void, a perfect meditative state, and Satori. It is generally thought of as gaining its value in the act of its creation, being a type of spiritual performance art, but they are also common in Buddhist art and are popular in commercial minimalist art in both the east and the west.