Enso is a Japanese word meaning "circle". Enso is perhaps the most common subject of Japanese calligraphy. Enso symbolizes enlightenment, strength, elegance, and the universe, and is an "expression of the moment".
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.
Some artists draw Enso with an opening in the circle, while others complete the circle. For the former, the opening symbolizes that the Enso is not separate, but is part of something greater.
The Enso is also a sacred symbol in the Zen Buddhist religion, and is often used by Zen masters as a form of signature in their religious artwork. See also Hitsuzendo for information about the Way of the Brush.
The Enso has also been co-opted as an advertising symbol by various companies, notably, Lucent. Lucent's logo is often jokingly referred to as the "coffee cup ring." Enso has also been co-opted by the company as the name of a series of software products based on the work of Jef Raskin meant to simplify common computing tasks.Wikipedia