Ouroboros is a symbol used in ancient cultures around the world. It consists of a snake biting its own tail, forming a circle, and like other circular motifs, it can symbolize unity or the cyclical nature of many aspects of life. The image is found in the cultures of ancient China, ancient Egypt, and the Americas, as well as in certain cultures of Europe and West Africa. Plato described it as the first living creature on Earth. Sometimes, the animal is depicted as a dragon instead of a snake.
Though the symbol of ouroboros is widespread, its meaning is not universal. Sometimes, it is meant to symbolize the joining of opposites, similar to the yin-yang, and is suggestive that opposites not only coexist but are also related in a cyclical manner. In this context, the image is sometimes depicted as half light and half dark. It was also used as the symbol for alchemy in the Middle Ages, as the goal of the practice was to join the conscious with the unconscious mind.
Ouroboros may be a symbol of eternity, infinity, and even resurrection, like many ring symbols. Conversely, it may be a symbol of the confines of this world, and when it was adopted into some regional sects of Christianity, it was used in this latter sense. It came to suggest a separation between the world, inside the circle, and infinite divinity without. The "self-devouring" nature of the snake likewise became interpreted as negative, as self-destructive and transitory rather than infinite. Whereas in earlier interpretations, the cyclicality was a disinterested observation about the nature of the world, in Christianity, it became more of a limited "vicious circle" that could only be broken through spirituality.
The image of a snake biting its own tail is a mysterious symbol that many people find compelling, even if its meaning is open to interpretation. It is often used in popular culture as a generic symbol of the occult, most often in the science fiction and fantasy genres. Fascination with the ouroboros also seems to transcend culture and religion, since it was such a widespread symbol in the ancient world. Much of this fascination seems to stem from the symbol's ability to represent many different things to different people.