Ouroboros

The circular image of a snake swallowing its own tail - an emblem of the eternal and indivisible, and of cyclic time. The image has been variously interpreted, combining the creation symbolism of the egg (the space within the circle), the terrestrial symbolism of the serpent and the celestial symbolism of the circle.

In its original Egyptian religious form, the ouroboros is thought to have symbolized the sun's daily return to its point of departure, passing through the sky and underworld. In Greece, death and rebirth symbolism seems indicated by its use in Orphic iconography. Gnostics saw it as an image of self-sustaining Nature, endlessly recreating itself, and of unity in duality, the essential oneness of life, the universal serpent moving through all things.

The maxim "One is all" sometimes accompanies the symbol. As an emblem of eternity, it was associated in the Roman Empire with Saturn, as the god of time, and Janus, the god of the New Year.

text from The Complete Dictionary of Symbols: In Myth, Art and Literature