"A brushed circle - the enso - is a traditional Zen calligraphic motif. It can mean many things: enlightenment, the present, completion... Just as one Zen philosophical tenet would be nowness, one important Zen aesthetic bias would be space. Sometimes you need to mark a point in space."
Powells Books - Essential Zen by Kazuaki Tanahashi and Tensho David Schneider

Enso - Wikipedia, the free encyclopedia