The Ouroboros, Greek for ‘tail swallower’ is depicted as a snake in circular motion swallowing it’s tail - on occasion, it can be seen in a lemniscate shape (as seen above).
It originated in Egypt to symbolize the sun; in terms of Gnostic beliefs, it is a referral to the solar god Abraxas, signifying infinity and the soul of the earth.
To those of alchemy it represents the spirit of Mercury (a substance that pervades all matter) and continuous renewal, life and death and the balance of opposition. A double Ouroboros (two snakes swallowing one another) reflects volatility, and spiritually it conveys the harmony of upper and lower natures.
It’s common symbolism that the snake represents being reborn, as it sheds it’s skin.
The Ouroboros is consistent throughout various cultures and settings - for example, the serpent Jormungandr of Norse legend encircled and protected the tree of life, or Yggdrasil. The Aztec snake god Queztacoatl was characterized similarly, and Chinese alchemical dragons share relative qualities both physically and in meaning.
(The background image is not mine, I don’t know who owns it so I can’t credit).