A circle is a closed flat curve made up of the points located at equal distance from a point called the centre. The value of this distance is called the radius of the circle. In a space of any dimension, the set of points placed at a constant distance from a centre is called a sphere.

The circle symbol has acquired several meanings. It is the perfect shape consisting of a point, its center, and its perimeter, an infinite line whose beginning is also the end. The circle embodies perfection, the infinite, the cycle, the absolute, the divine and the spiritual. It is used to represent the world and what the world revolves around. The circle or sphere also represents the enclosed space, the cocoon, fertility, the nurturing mother, the centre (of life).
Finally for many art theorists the circle is often associated with colours or numbers. For Kandinsky or the Bauhaus school the circle was associated in their correspondence of colours to blue.

From the time of the earliest known civilisations – such as the Assyrians and ancient Egyptians, those in the Indus Valley and along the Yellow River in China, and the Western civilisations of ancient Greece and Rome during classical Antiquity – the circle has been used directly or indirectly in visual art to convey the artist’s message and to express certain ideas. However, differences in worldview (beliefs and culture) had a great impact on artists’ perceptions. While some emphasised the circle’s perimeter to demonstrate their democratic manifestation, others focused on its centre to symbolise the concept of cosmic unity. In mystical doctrines, the circle mainly symbolises the infinite and cyclical nature of existence, but in religious traditions it represents heavenly bodies and divine spirits. The circle signifies many sacred and spiritual concepts, including unity, infinity, wholeness, the universe, divinity, balance, stability and perfection, among others. Such concepts have been conveyed in cultures worldwide through the use of symbols, for example, a compass, a halo, the vesica piscis and its derivatives (fish, eye, aureole, mandorla, etc.), the ouroboros, the Dharma wheel, a rainbow, mandalas, rose windows and so forth.
– Jean-François Charnier, “The Circle from East to West”.