Rassilon Imprimatur

Rassilon Imprimatur is linked to (or a part of) the symbiotic nuclei which allows a Time Lord access to time travel and in particular temporal vessels such as a TARDIS.

TARDISes (or other time machines) must be primed with a Rassilon Imprimatur to enable them to travel safely in time. (DW: The Two Doctors)

The Imprimatur maps Time Lords onto the time vortex as pure mathematics. (EDA: Interference - Book One)