A door was an architectural feature blocking or allowing access to another space.
The Doctor's TARDIS, as used by the First Doctor, had two doors, the outer door of the exterior entrance chamber, followed by the inner door which led to a fourth dimension, beginning with the control room. (COMIC: The Secrets of the Tardis)
Trying to kill the Third Doctor, the Master attached a volataliser to a door at the Deep Space Research Centre No. 2, so that if anyone tried to open the door it would fall to the ground and destroy the building. The Doctor was able to act quickly enough to catch it before it detonated. (PROSE: Doctor Who and the Terror of the Autons)
The access code to open the door from the Chancellor's office to the President's office on Gallifrey was the spoken phrase "there's nothing more useless than a lock was a voice print", a popular saying of Borusa's, were spoken. (TV: The Invasion of Time)
When an intelligent Earth species reached the edge of the Solar System, they and their entire timeline were sucked through a door into Time's End. This happened to billions of species until it was stopped by the Sixth Doctor. (AUDIO: The Nowhere Place)
The Twelfth Doctor was trapped in the TARDIS when it shrank so much from the Boneless' influence that he could not fit through the door. (TV: Flatline) This same incarnation of the Doctor later remarked that it was impossible to form a bond with a door, as they were "notoriously cross" with their lives, such as they were, consisting of people "pushing past them". (TV: Heaven Sent)