Ouroboros

The Ouroboros (Greek Οὐροβόρος or οὐρηβόρος, from οὐροβόρος ὄφις "tail-devouring snake", also spelled Uroboros in English pronounced /ʊˈɹɒbɔɹɔs/ or /ˌjʊəɹoʊˈbɒɹəs/), is an ancient symbol depicting a serpent or dragon swallowing its own tail and forming a circle.

The Ouroboros often represents self-reflexivity or cyclicality, especially in the sense of something constantly re-creating itself, the eternal return, and other things perceived as cycles that begin anew as soon as they end (See Phoenix). It can also represent the idea of primordial unity related to something existing in or persisting from the beginning with such force or qualities it cannot be extinguished. The ouroboros has been important in religious and mythological symbolism, but has also been frequently used in alchemical illustrations, where it symbolizes the circular nature of the alchemist's opus. It is also often associated with Gnosticism, and Hermeticism.

http://en.wikipedia.org/wiki/Ouroboros