Inner door
From Tardis Wiki, the free Doctor Who reference
The inner door was the second of the two doors of the TARDIS, as used by the First Doctor, following the outer door. The inner door was at the rear end of the exterior entrance chamber, leading to a fourth dimension which began with the control room. As the Doctor noted to Ian Chesterton and Barbara Wright, a reversal of time acceleration occurred at the inner door, explaining how the TARDIS was bigger on the inside. (COMIC: The Secrets of the Tardis)
