Drawing by Theodoros Pelecanos, in an alchemical tract titled Synosius (1478).
The Ouroboros or Uroborus 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 (compare 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.
Old alchemical woodcut of the seal of Solomon with an ouroboros.
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. (from Wikipedia)
Serpent feeding on its own tail ~ Michael Maier, 1618
The circle is the symbol for infinity, because the circle is endless, and may be considered a polygon with an infinite number of sides.
The snake or dragon with its tail in its mouth continually devouring itself and being reborn from itself is a symbol of eternity and of the cyclic nature of the universe. It expresses the unity of all things, which never disappear but change form in a cycle of destruction and re-creation.
It is also the alchemical symbol for chemical change. A dream about this serpent gave the chemist von Stradonitz the notion of the benzene ring, in the 19th century.
Ouroboros by Zarathus