Dematerialisation circuit

A demateralisation circuit was an essential part of a Type 40 TARDIS which enabled it to de-materialise from normal space into the Vortex and re-materialise once more. During his exile on Earth, the Time Lords removed the "dematerialisation codes" from the Doctor's mind. Once they had rescinded his exile after his defeat of the renegade Time Lord Omega, the Time Lords restored this knowledge to the Doctor memory. (DW: The Three Doctors)