The circle is simultaneously everything and nothing.
A circle leads back into itself and consequently is a symbol of unity or the absolute. As a result it often makes reference to the immaterial rather than the concrete or the spiritual in contrast to the material. The circle also symbolizes time and infinity often in the form of a serpent eating its own tail. The completeness of its form most often represents wholeness, or the eternal and self contained. The boundary of the shape denotes the parameters of time enclosing space as well as timelessness, having no beginning or end and spacelessness with no above or below. The spherical in its infinite aspect becomes the abolition of time and space while also signifying reoccurrence.
As a disk it is used to symbolize the sun and solar energy as well as the ascent to higher consciousness. As representative of the sun it is the manifestation of creative revolving energy celestial unity, the solar cycles, dynamism, endless movement, completion, and fulfillment.
The circle is a paradoxical symbol: its complete bounded area can be viewed as a limitation or boundary; however, it is this same completeness, moving one round and round that releases its symbology into the infinite. It is the unity of its border demarcates all that exists; we provide ourselves with a circle of reality and it is through the wholeness of its walls that we are able to see beyond their confines into infinity. The boundary of the circle can be used to set apart sacred spaces and the internal space of Self.
The circle as the infinite, often represents the feminine (birth, life giving, Maya) as opposed to the linear masculine potential. The circle is the round of existence enclosing all that is within the phenomenal world. The confines of the circle are the bounds of ignorance; the empty circle or the void is representative of transcendence or enlightenment. A circle with a dot in its center depicts a complete cycle and cyclic perfection, the resolution of all possibilities in existence. The central point is the supreme place of all knowing, the point of stillness from which all is seen in truth.
From "The Work" by John Merideth 1994