Seal of Rassilon

The Seal of Rassilon was a symbol used by the Time Lords.

Use by the Time Lords
The seal was a symbol of power, utilised by Rassilon during the war against the Great Vampires and Old Ones (EDA: Interference - Book Two).

The Seal was used as a symbol of identification, and was viewed with deep reverence. (DW: The Five Doctors, Doctor Who) Time Lords displayed the Seal of Rassilon in places of great power. (DWN: Doctor Who - The Novel of the Film) The Seal was said to be an ancient symbol for warding off evil. (NA: The Pit)

When he sent the Master 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 Doctor of the Master's good intentions. (DW: The Five Doctors)

At some point, the Time Lords also used a different seal, bearing Gallifreyan markings. (DW: The Sound of Drums)

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, such as the inhabitants of Voga. (DW: Revenge of the Cybermen, NA: Cat's Cradle: Time's Crucible)

Occurrences
Known objects to be marked with the Seal of Rassilon include:
 * Badges or embroidery on Time Lord collars or robes, usually worn in pairs. (DW: The Deadly Assassin et al)
 * The Fourth Doctor's signature to a letter written in Gallifreyan (DW: The Deadly Assassin)
 * The Seal of the High Council (DW: The Five Doctors)
 * The Seventh Doctor's TARDIS key (DW: Ghost Light)
 * Various areas throughout the Eighth Doctor's TARDIS, most notably embossed on the Eye of Harmony itself. (DW: "Doctor Who")
 * The payment box for the Threshold. (DWM: Fire and Brimstone)
 * The Worshipful and Ancient Law of Gallifrey (WC: Shada)
 * Shada itself (WC: Shada)
 * The Untempered Schism (DW: "The Sound of Drums")
 * On the notes scrawled by The Visionary (DW: The End of Time)
 * On the front of the Eleventh Doctor's TARDIS-blue diary of his meetings with River Song (DW: "The Impossible Astronaut")

Behind the scenes
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.