Seal of Rassilon

The Seal of Rassilon, also called the Prydonian Seal or the omniscate, was a symbol used by the Time Lords.

Use by the Time Lords
The omniscate was a symbol of power created by Rassilon during the Vampire Wars. (PROSE: Interference - Book Two) It was designed to ward off evil, (PROSE: The Pit) and it was specifically repulsive to creatures from outside the universe, like the Great Vampires. (PROSE: Interference - Book Two)

The Seal was also used as a symbol of identification, and was viewed with deep reverence. (TV: The Five Doctors, Doctor Who) Time Lords displayed the Seal of Rassilon in places of great power, like the Eye of Harmony. (PROSE: The Novel of the Film) Each Time Ring was stamped with a swirling emblem that resembled a twisted ouroboros. (PROSE: Warlords of Utopia)

When he sent into the Death Zone to rescue the Doctor, Lord President Borusa gave the Master the "Seal of the High Council", a round metallic object emblazoned with the Seal of Rassilon, hoping it would convince the Third Doctor of the Master's good intentions. The Doctor promised to return it at the first opportunity; however, he never did so. (TV: The Five Doctors) The Eleventh Doctor later attached this same seal to Handles to use an algorithm in the seal's atomic structure to decode a message from the Time Lords. (TV: The Time of the Doctor)

At some point, the Time Lords also used a different seal, bearing Gallifreyan markings. (TV: The Sound of Drums) During the War in Heaven, the House Military used a version of the sigil that had a sharp-edged, aggressive shape imposed upon it. (PROSE: Warlords of Utopia)

Use by other cultures
The symbol was also taken up by other cultures whom the Time Lords visited early in their explorations of space and time, (PROSE: Cat's Cradle: Time's Crucible) such as the inhabitants of Voga. (TV: Revenge of the Cybermen) The Order of the Garter's garter symbol resembled the ouroboros at the center of the Great Houses' sigil. (PROSE: The Book of the War)

Dana MacLeish had a tattoo of the symbol on her back. (COMIC: World Without End)

Occurrences
Known objects to be marked with the Seal of Rassilon included: badges or embroidery on Time Lord collars or robes, usually worn in pairs; (TV: The Deadly Assassin et al) the Fourth Doctor's signature to a letter written in Gallifreyan; (TV: The Deadly Assassin) the Seal of the High Council; (TV: The Five Doctors, The Time of the Doctor) the Seventh Doctor's TARDIS key; (TV: Ghost Light) various areas throughout the Eighth Doctor's TARDIS, most notably embossed on the Eye of Harmony itself; (TV: Doctor Who) the warship containing The Cold; (PROSE: Interference - Book Two) the payment box for the Threshold; (COMIC: Fire and Brimstone) The Worshipful and Ancient Law of Gallifrey and Shada; (WC: Shada) the Untempered Schism; (TV: The Sound of Drums) the notes scrawled by the Visionary; (TV: The End of Time) the Eleventh Doctor's TARDIS-blue diary of his meetings with River Song; (TV: The Impossible Astronaut) various areas of the Eleventh Doctor's second TARDIS interior; (TV: The Snowmen, et al.) the front cover of the book The History of the Time War; (TV: Journey to the Centre of the TARDIS) and the armour of the Time Lord soldier (TV: The Last Day, The Day of the Doctor, Hell Bent) and the General. (TV: The Day of the Doctor) The Doctor's daughter, Jenny, also wore a body suit emblazoned with multiple Seals of Rassilon. (COMIC: The Lost Dimension)

Behind the scenes

 * Sketches of the Seal of Rassilon by John Smith, accompanied by notes suggesting that the symbol is meaningful and important for unknown reasons, appear in pages of the A Journal of Impossible Things prop not seen on-screen. They are viewable in the replica sold by Character Options.
 * The Seal's introduction as a Time Lord symbol in The Deadly Assassin came about when designer Roger Murray-Leach reused the Vogan circular motif from Revenge of the Cybermen, on which he had worked two years previously. The symbol retained its association with the Time Lords and Gallifrey, and became known as the Seal of Rassilon thereafter. Cat's Cradle: Time's Crucible then retconned the Vogans' use of it to be a reference back to the Time Lords.