A non-definitive (and very brief) history of the Ouroboros
How to pronounce: Ouroboros ("OOR" + "uh" + "BAW" + "ros")
This article originally published in 2014 (links updated)
In my novel, The Ouroboros Key, the symbol is the Ring of Unity created by Tiamet and Enki to aid in bringing unity to the world of humanity.
Wikipedia will tell you that the Ouroboros originated in Ancient Egypt and represented cyclic recreation.
We also know the Ouroboros as the Ring of Unity, which we first hear about in Ancient Sumeria (read Realm of the Ringlords: beyond the portal of the twilight world by Laurence Gardner).
The ring is depicted as a snake or serpent holding its tail in its mouth.
This same “ring” (according to the late Laurence Gardner) is the basis of most ring-lore in legend and history including references in The Lord of the Rings books by JRR Tolkien and The Ring Cycle by Richard Wagner.
Since I wrote the above, lots more research has become available so here's a 2021 further reading list:
The ancient symbol that spanned millennia