Enso (円相)

Enso is a type of calligraphic technique in Zen calligraphy, which is a circle painted with a single stroke.
It is also called as 'Ichienso' or 'Ensozu.'

It is thought to be a symbol of enlightenment, truth, bussho (the Buddhist nature) or the whole universe, but the interpretation is left to each person.

Also, sometimes the Chinese characters 円窓 is used for enso, which means 'a window that shows your heart.'