Ouroboros

The Ouroboros was the symbol of a snake or worm (PROSE: Falls the Shadow) eating its own tail, representing eternity. (AUDIO: The Tears of Isis)

The sigil of the Great Houses included an ouroboros, as did the garter symbol used by the Society of St George. (PROSE: The Book of the War) On Earth, the origin of the symbol was the Demesne Furze, the Great White Wyrm worshipped by the Romans. (AUDIO: Trail of the White Worm)

The signature emblem of the Corsair was an Ouroboros. According to the Eleventh Doctor, he had the image tattooed on his body after each regeneration, even when he regenerated as a female. Without it, the Corsair didn't feel like him/herself. (TV: The Doctor's Wife) Jan Rydd had an ouroboros tattooed around his neck. (PROSE: Love and War)

The Mara was described as ouroboros, (PROSE: Antony and Cleopatra) as was the Timewyrm which was said to circle the universe and bite its own tail. (PROSE: Timewyrm: Revelation)

Concepts named after the symbol included the Oroborus entity, (TV: Oroborus) the Oroborous Worm, (PROSE: The Mary-Sue Extrusion) and the Ouroboros loop. (AUDIO: The Tears of Isis)