The enso, a simple circle drawn with a single, broad brushstroke, is the
Zen symbol of infinity. It represents the infinite void, the 'no-thing', the
perfect meditative state and Satori (enlightenment). Enso is a Japanese
word-meaning circle. Enso is perhaps the most common subject of Japanese
calligraphy. It is believed by many that the character of the artist is
fully exposed in how he draws Enso and that only one who is mentally and
spiritually whole can draw a true Enso. Some artists will draw Enso
daily, as a kind of spiritual diary.

The Enso is a sacred symbol in the Zen Buddhist religion. It is often
used by Zen masters as a form of signature in their religious artwork,
symbolizing enlightenment, strength and the universe. Enso is an
"expression of the moment."
References
http://en.wikipedia.org/wiki/Enso
http://altreligion.about.com/library/glossary/symbols/bldefsenso.htm