The Ouroboros, or the "tail-devourer," is the symbolization of concepts such as completion, perfection and totality, the endless cyclical nature of existence. It is usually represented as a worm or serpent with its tail in its mouth.

This symbol appears principally among the Gnostics and is depicted as a dragon, snake or serpent, biting its own tail. In the broadest sense, it is symbolic of time and the continuity of life. It sometimes bears the caption Hen to pan  - 'The One, the All', as in the Codex Marcianus, for instance, of the 2nd century A.D. It has also been explained as the union between the chthonian principle as represented by the serpent and the celestial principal as signified by the bird (a synthesis which can also be applied to the dragon).

The Ouroboros, biting its own tail, is symbolic of self-fecundation, or the primitive idea of a self-sufficient Nature - a Nature, that is which, continually returns, within a cyclic pattern, to its own beginning. There is a Venetian manuscript on alchemy which depicts the Ouroboros with its body half-black (symbolizing earth and night) and half-white (denoting heaven and light).